So, real talk? Most 2600 games are rough, and barring personal nostalgia, there's little reason to play most of them in the age of emulation, especially arcade conversions, which sometimes nail the gameplay (but often don't), and generally have to perform acts of violence on the visuals to make them work with the system and the business realities around their development (i.e. staffing, timeline, budget for ROM chips, etc.).
Some worthwhile ones that come to mind:
Combat (multiplayer only)
Warlords (multiplayer only)
Pitfall
River Raid
Pitfall II
Space Invaders
The Empire Strikes Back
It's not that so many more weren't fun, or even still aren't in isolation, but it's like we're all the rich fat kid from Pee-Wee's big adventure and have access to every single game on every single system, at least up until the end of the 90s. There's no reason to play the nice port of Berzerk that looks like it does, or play the flickery Pac-Man mess, or even (I'll say it) fight with the groundbreaking but still primitive and abstracted gameplay of Adventure.
I wouldn't presume to have even 20% of the 2600 games that bring something different and good to the table, it's just (to misquote Samuel Johnson), for so many of them the good parts are not different and the different parts are not good.
And again, that's completely apart from a personal nostalgia (god knows I indulge in that) or to propose that they're simply not fun in a binary sense. If I'm 12 and I get 2600 Venture I enjoy the hell out of it, but if I'm a middle aged man in 2024, at the bare minimum I'm going for this.
Combat and Gorf are amazing. I may feel that way about Combat because it was the first one I remember playing. However, Gorf is like three games in one so you got a lot of bang for your buck in playtime.
The Degenatron gaming system plays three exciting games, including Defender of the Faith, where you save the green dots with your fantastic flying red square.