ODST without a doubt. Great cast, great story, great atmosphere, great balance, great game.
I also really liked how they built all of the standard videogame level tropes into the story so well. Typically it feels a little forced how you just happen to be in a "vehicle level" or a "sniper level" or a "stealth level". They might fit into the story, but it often feels a little too convenient. I feel like ODST did a really nice job of using the specialised squadmates to lend some weight and reasoning to structuring the game with specialised levels like that.
And I also thought they managed to do the overworld mission hub thing really well with you looking for the remains of your squadmates' battles. It was an elegant way to do a mission hub again without it seeming too artificial or forced.
But most of all, the voice cast (i.e. the cast of Firefly) was great and the darker atmosphere was very welcome. The fact that it wasn't about MC and wasn't part of such a giant, epic story also helped make the more impressive skirmishes that you actually take place in seem that much neater - it didn't feel like a lot of the game was just filler between larger, bigger, cooler battles.
Reach was also very, very good. I think it was a really nice blend of the things that worked in ODST and the characters and themes of the main trilogy. And it had some damn impressive set pieces. My only real complaint was that I wished I could fly around that fighter in space more!