I had Halo CE on Xbox, on release. I'd played a ton of FPSes on the PC, but Halo was like a breath of fresh air. The graphics (those who played on PC missed out, that port looked like shit), sound, music, dialogue of your enemies and marines. Even the AI was pretty good, in the big open vehicle sections, you could play them a variety of ways. I loved it.
Halo 2 completely dropped the ball for me, what a disappointment. The graphics pop-in issue was pretty horrible, clearly the game was just overambitious in scope. New enemy types were just not fun to fight, the fucking jackal snipers? Brutes are lame compared to elites. The story felt all over the place. There's no level i would play again in 2, but I'd play Silent Cartographer, AotCR and Two Betrayals over and over.
3 attempted to get the magic back, 4 player co-op was fun, but again I can barely remember it because I didn't want to play it again after completing it. The only positives I get from 2 and 3 are the music.
ODST feels a lot more like Halo CE to me and I liked that. I only played through half of Reach, a few years after release, because I thought the art style looked really bad in trailers. But when I got round to it I thought it was ok. I haven't played the other games except for free Infinite multiplayer.
So yeah for me Halo peaked at CE and it's been a dead franchise for a while. Mulitplayer in 3 was great I cant deny, but only because I could play with 3 other people on the same tv against other people doing the same thing and voice chat with them.