I'm really enjoying this game, so far at least (guesstimating half through the campaign). Quite a bit better than Halo 3 imo, which in my book would make it second best to Halo 1 I guess in the rankings.
I think a lot's to do with expectations, Halo 3 was a huge proper Halo game with all the hype, debut on a new console generation, I felt it was a letdown. However ODST, I knew going in it was Halo 3 ++, and I wasn't expecting much, and it turned out to be a nice little game. Really nice in fact at this point.
I really feel it's a bit of a return to form for Halo for a lot of reasons, and it also doesn't feel like any other FPS with it's hub world>flashback missions structure, and I actually like it a lot, whereas I thought it sounded quite stupid at first. I also think the storytelling is better. At first I kinda wanted to laugh at Bungie over the whole fact this is essentially Halo as a 40's film noir mystery, no kidding, but it actually works pretty well. The atmosphere in the hub city is great as well as Marty O'Donnell's music as always. At first I thought I would hate the city being at night the whole way as it somewhat hampers exploration of course, but now I dont mind.
I also, gasp, think it looks pretty good, and I've beaten Killzone 2, played a lot of Crysis PC maxed, etc.
I havent touched Firefight yet (I'm not big on horde modes anyway).