I like Reach for the armour variants. The game itself... It's alright. Everyone seems to have gone nuts over the Reach multiplayer and for so many younger Halo gamers that is 'the start and pinnacle' of Halo and was a bandwagon hubby and myself didn't get on. Technically storyline wise it is, but you cannot beat Combat Evolved. That is THE Halo.
I personally like ODSTs firefight, that's good fun
What I like about Reach is the perks (abilities ?) that you can get. Especially the Jet Pack.
Firefights are the most common thing that I do on ODST. I also play infected on there quite a bit.
I have a friend who is crazy good at any Halo. It is almost not even fun playing with him because he is so good. He literally gets headshots every single time.