Best Frames for Steel Path?

If you ask me, "best frame" is nothing if you don't like playing it in the first place. Do you like tanks? Casters/ability damage frames? Weapon platform frames? There's also many variables introduced via Helminth. And the best frame won't do shit if you don't mod it right and don't play it right the higher you go - higher levels increase reliance on shield gating, staying mobile and things that let you straight up ignore damage.

Here's some things I do like a lot to go for myself though but I can take most frames currently and get them going for a long time in SP:

-Mag - very easy shield gating, elevates projectile weapons to great heights

-Zephyr - can ignore most damage, has incredible crowd control coupled with damage amplification

-Xaku - yoink gun be skelly, full defense strip and scaling damage

-Nezha - can tank for a very long time and isn't hard to make into someone who can tank and also shield gate

-Yareli - slightly more gimmicky water Nezha

-Ivara - when I want to snipe heads, camp a spot or roll around

-Gyre - what if World on Fire but electric and you can use Pillage to make it never fall off and survive too

-Grendel - UWU. Can shield gate, can tank, can strip armor.

-Citrine - well I'm a big status stan and she got this plenty

-Baruuk - punch, tank, with Pillage can go 5ever

Revenant is immortal, can be built for literally deleting health bars or for being an immortal platform for weaponry. Gauss, a zoomies full package boi. Gloom Banshee keeping enemies crowd controlled, coupled with armor strip and incredible damage potential. Garuda with endless slashes. Ash with his Disruption popularity.

And then there's Octavia which I can't play for long before I get bored off my ass but she's able to go ad infinitum especially if you give her an armor strip.

Condemn Banshee is my fav. Fullstrip, massive damage vuln, great CC, your friends will love you. Pillage Gyre is amazing if you invest some red shards in her, mine's currently sitting at 430% strength.

Even with endurance your "best" picks depend on which mission type you're doing specifically. Your Survival aoe DPS frame won't really cut it in a long Disruption, and your single target CC Disruption frame won't really cut it in a long Void Cascade, etc

In a Survival you get the most flexibility as you only need high enough KPM to keep up life support, so it's hard to suggest the "best" frame. You'll want DPS to be your main focus, so frames like Khora, Garuda, Saryn, Mag, Baruuk are great choices. Frame survivability can be easily solved with a bit of CC and using the environment - find a corridor or a side room for a camping spot to limit enemy LOS and gunfire. If going beyond lvl 1000 (which is a good few hours in) you'll be mostly forced into shield gating builds, but health/armour tanking and damage reduction can get you pretty far, so you can use something like Nidus for a good long while without too much stress

In Disruption, the things you have to play around are the energy drain and enemy Toxin weapon Conduit modifiers. Both of these absolutely fuck over shield gating builds, and once you're far enough in, health/armour tanking falls off anyways. You can brute force through both of the modifiers with Vazarin dashes, but if you want safe and reliable picks that don't rely on Vazarin, then Revenant and Zephyr shine for their Mesmer Skin and Turbulence. Stealth frames like Ash or Loki avoid all risk of enemy Toxin weapons (other than crossfire), and can Vazarin dash through an energy drain round to drop an energy pad at the Demo. If you're confident using Vazarin to cover you, you can use literally anything, and some great picks are frames like Xaku and Mag. But being confined to Vazarin can restrict build variety a bit (like using Unairu for armour strip)

In Void Cascade, you have an extremely fast mission objective you have to keep up with, while being constantly harassed by enemies. You can't set up a safe camping spot like Survival, and you can't set the pace like Disruption. You can bring anything with enough damage output to deal with Thrax and trash enemies, but since the mission dictates where you have to be, you can often find yourself in places with little to no cover, and it's very demanding on shield gating builds. This again means that Ash, Zephyr, and Revenant are your "best" picks for survivability and keeping on top of the cycling objective. You can bring Vazarin, but it doesn't bring any damage buffs like Madurai or Unairu to help with killing Spectral Thrax, so when trying to put together a build that covers all bases you can find yourself much more restricted than in Survival or Disruption

Garuda, love the invicibility with 4, constant power regen with 3, scalable damage to enemy level with 1, subsume 2 as you'll have the mod that gains you life when you spend energy. Innate slash damage. Bump up cast speed, become unstoppable in a room full of enemies. I do love playing as Atlas with the free punch augment tho, but struggle to pay enough attention to rubble.

Octavia is one of the strongest and most AFK frames you could get. Every time I get her in the SP circuit, I know it’ll be an extremely easy couple rounds until I get bored due to it being such low input. Absolutely nuts

Revenant is the king of not dying, and while that's not all that much it is all you need to succeed in endurance if you can mod weapons well. He is the most relaxing and least-demanding option. His 1 and 3 combo to deal percentage damage to enemies, so they scale infinitely. There's no better frame for endurance, though he's not that great of a pick outside it.

Alternatives include any frame with invisibility.
Loki gets both invisibility and invincibility courtesy of Safeguard Switch, so he's one of the safest frames to play provided you have your timing down well (or just spam your switches).
Ivara has to handle prowl's limitations, but if you're able to (You can use power strength to boost her base movement speed with her augment, and archon shards to increase the speed of her roll with the parkour velocity stat) she can be a devastating killer.
Ash is a complex mess. He is both simultaneously great and terrible, mostly because he has an overwhelming need for augments to patch up his poor base kit. Note that Bladestorm carries Ash hard, and is an ability that has (some) damage that scales off your melee in a very strange way. AznvasionsPlays made a video on it here and the ability will still handle endurance without the bane nonsense.
Voruna isn't a terrible choice, provided you can handle either recasting her 1 after every single heavy melee attack for a surprise melee build or manage a pounce build using her 2 and 4. The former requires you have a very good weapon for heavy melees that can wipe groups in single swings, which isn't too hard to acquire but does mean you need to know especially what you're doing. The latter is easier but does require quite a bit of setup and can be especially slow at times due to the RNG of her 2. Basically, you use her 2 as a primer and her 4 to knock things down. The latter build is expensive, though, as you'll need very good energy economy either from yellow archon shards or arcane energize (as her 3 is completely useless in this build, subsume over it with Roar, you'll need the double-dipping damage boost).
Octavia is known as the queen of... well, everything. Provided you can handle spamming your crouch a bunch, setting her music to hit every beat means you will (almost) always have the buffs racking up on accident, and thus will always be invisible. Combine that with her mallet that scales off enemy damage when they target the thing and she can easily clear just about any threat as long as it touches the mallet and not her.

As for good shield-gaters, there are quite a few.
Harrow is an obvious option, but he can be quite difficult to play at high levels.
Hildryn is another option, but she can have issues between balancing ability strength to full-strip with pillage and managing her shield to not die (especially against infested). Not a terrible option by any means, but you will have to decide between full-stripping and having issues running Hildryn with too many allies (as they will steal all your shields via Haven) or doubling the pillage casts needed each mission. Wisp is a decent option. She's technically an invisible frame, she's invisible as long as she doesn't attack and doesn't touch the ground (or as long as her 2 is active) but she's effectively a shield-gater since you will need to attack at some point and her 2's normal use in endurance is to grant you invincibility at the end of it, not the invisibility during it. For obvious reasons, her health motes are largely useless at such a stage in the game.
Protea is an alright option. Personally, her 1's shield refresh is a bit slow to cast and she doesn't do amazing damage with blaze artillery (but they are heat so they'll function effectively and kill things quick enough) but she can do endurance better than a lot of frames.
Volt is a good option. Capacitance allows him to charge his shields effectively all the time, so he's actually one of the better shield-gaters provided he's well built.
Garuda is also an option. Combining her 4 (which grants invincibility during casting) with Wisp's 3 as a subsume allows them to chain off each other, creating a slow run-away reaction that can kill everything and everyone in the vicinity. Using her 4, in-built 3, and Molt Reconstruct means she essentially has all the energy needed and all the health to supply that energy, and her 4's invincibility period combined with augur mods will let her shield-gate effectively.

As for the special snowflakes (other than Revenant), there's Zephyr and Valkyr.
Zephyr is invincible provided three things: You build range, always keep Turbulence up, and never let anyone melee you. Building range on Zephyr is the easiest thing to do, every ability of hers wants it and she can easily drop strength. Keeping Turbulence up is as easy as Natural Talent (if you even need that) and not forgetting the timer. Never letting anyone melee you is as easy as holding her 1, which will let you float in the air as long as you have energy. Now, unfortunately there are a few things that do get past Turbulence these days (cough Thrax magnetic beams cough) but you should be able to easily survive even against those kinds of things if you keep your wits about you (and have Rolling Guard).
As for Valkyr, she's the immortal kitty who cannot die so long as she has energy. That's pretty easy to accomplish, but the real issue is now you have to use her meh exalted weapon instead of the numerous better things you could be using. She's not my first pick for anything, but she works.

Nezha/Rhino are pretty easy mode, Rhino can get some pretty silly ironskin numbers into the 300-500k with the right build. While Nezha is pretty similar. Nezha gives a huge damage vulnerability to enemies he hits with his ring, while Rhino gives a damage increase to all of his damage. Nezha gives more, but affects enemies (so less) while Rhino buffs himself. Rhino is a bit more microintensive since you need to use his ironclad charge.

Xaku is pretty good but as he only has dodge chance he can end up dying out of nowhere. You have to play a bit more quick for when your shields break but he has defense strip, scaling damage, the ability to kill enemies while not looking at them, and his 4 makes enemies weak to void damage which his scaling damage (stolen guns) do.

Zephyr probably is the easiest/best one (but also a bit boring), she can ignore all damage while grouping all enemies and dealing damage to every single one. While also increasing her crit chance.

Citrine is really fun has she has a unique build that not many frames use, instead of using flow you run equilibrium and synth fiber to infinitely pick up health orbs, which constantly gives her energy. As her 1 makes enemies drop health, it's an infinite loop. She also has 90% DR constantly due to killing to her entire team in affinity range, and she can give a huge critical buff when she stuns enemies and you shoot the crystals sticking out of them.

Revenant is pretty much is unable to die or take damage, and he can kill his thralls (who can be any enemy except eximus with overguard) with a 50% health drain that is unaffected by armor. So he can technically kill enemies to level cap. As long as you keep his mesmer skin up, nothing can harm you.

Revenant, Protea, Citrine, Styanax, Kullervo can all handle over an hour of Steel Path easily without excessive setups or mechanic abusing while also being engaging and fun to play.