3 is a big return to form (but not a return to the .45 magnum (that came with Reach)) in that it is much more about sandboxes where you (and three friends) are given your choice of tools. Much less authored than CE/2’s campaign structure and has aged wonderfully as a result. Multiplayer is impossibly well balanced and its legendary plate-spinning haunts the people responsible for maintaining balance simultaneously across PvE and PvP in Destiny to this day.
ODST is stretching tone and narrative muscles they may not have used since Marathon (taking @shrug’s word for Marathon’s prowess here, can’t into keycards, born in '92) with an espionage story with a neat twist that is basically the antithesis of Starship Troopers (Verhoeven, again). There’s a song with a soprano sax solo and another song that’s basically Kashmir by Led Zeppelin and they somehow cohere. It’s a great gaiden game, made with Halo 3 assets as a series of flashbacks you access by finding your teammate’s gear around a hub city you can explore freely. Definitely prototyping for Destiny at work here. They also got a bunch of Firefly actors and use them well (?!).
Firefight is unlimited PvE Halo and introduced in ODST/modified in Reach. Best Horde imo, just lots of fun doing improbable bullshit around the margins of the sandbox.
I must admit I am a hardcore Reach stan - got the legendary solo campaign achievement.