I love Neal Asher's books, found him a long time ago in one of those "year's best" collections of short stories from the library (though the ones with fantasy and horror were always the best, I think I read every single collection for every year and found so many good writers that way.)
They are full of action, good characters and worlds and ideas, sweeping and huge settings. Feels almost more like watching a movie to read them.
Space Opera has got to be my favorite genre, but it feels like we have so precious few that actually nail it. Mass Effect, and Star Wars are ones that instantly come to mind, Starship Troopers is an excellent piece that also falls into the same vein.
There’s a close relationship, or at least some significant overlap/intersection between military SF and space opera.
Weber’s Honor Harrington series would be considered both. Likewise, Tanya Huff’s Confederation novels are unabashedly military SF, but within a decidedly space opera overall frame that is progressively revealed through the course of the series.
Some space opera series, like Bujold’s Vorkosigan Saga cross back and forth across diverse sub genres - some military SF in some, political space opera in others along with some social and genetic engineering here and romance there.
CJ Cherryh’s Alliance-Union Universe isn’t primarily military SF, but it’s in the mix.
The Quantum Magician and The Final Architecture are great space operas in my opinion. They both have really large stakes, a large number of factions and species with varying politics and exciting space combat. They are both incredibly different though.
Some of my all time favorites are the Spinward Fringe series by Randolph Lalonde, The Intrepid Saga from the Aeon 14 universe, and the Imperial Radch Trilogy from Ann Leckie.
A Memory Called Empire was really good, but it wasn't a space battle shoot-em-up, more like large scale political intrigue and murder mystery.
@Izzy brought up Tchaikovsky's Final Achitecture and @ScrumblesPAbernathy mentioned Children of Time series, loved those. Right now I'm on the second of his Bioforms books, Bear Head. After that it's back to Idris and The Lords of Uncreation.
While it probably doesn't qualify as space opera, I have to throw in The Murderbot Diaries by Martha Wells. I just fucking love Murderbot and ART.
There's always the Bobiverse, The Culture Series, and The Cluster Saga, too.
The Vorkosigan Saga by Bujold is considered to be some of the best space opera in the entire genre, definitely recommend giving it a try. Start with Warriors Apprentice if Shards of Honor isn’t to your liking.
I jived best with Warrior's Apprentice, but my friend, who reads more romance (I generally don't read it), liked Shards of Honor just fine.
The series as a whole is top-tier sci-fi, but Shards of Honor is definitely a romance in SFF trappings, and one of Bujold's earlier books (which, to me, shows, as she's still honing her chops).
My favorites are distributed around, and I tend to read in publish order so I have to power through some of the slower ones. They're insanely good, though.
For shows, Battlestar Galactica is always great. I just started watching Caprica and loving it so far. It might not be an “opera”, but it does so much to fill out the Battlestar Galactica universe.
I also just started reading The Expanse series and would definitely recommend it, even for someone like me who’s not an avid reader.
For me, the favorite of the genre to date are the Mass Effect games, Star Wars (the good ones, at least), Asimov's Foundation books, Stanislav Lem's Solaris book and, even if it's more grimdark than space opera, Warhammer 40k. I still have to delve into The Expanse but I don't have the time yet
I found I clicked more with The Expanse as the TV show than the books.
Some series are like that for me. I read so much more than I watch shows, but some stories I don't click with until they end up as a movie or TV series.
Consider Phlebas is a favorite of mine for this reason. Two agents on opposite sides of a war trying to reach the same place and capture the same thing. The climactic battle at the end is amazing, and not just because I love trains