Halo 4 by far, but it's still not "amazing"
Halo Reach just wasn't fun in my opinion, it didn't flow too well and I hit max rank in it.
I'd say Halo 3, if you can find a game in it then you're going to have a blast.
I personally feel like Reach is better. Halo 4 feels like an alien COD, with the killstreaks and how you can create your class and everything. Who knows, maybe you're into that. If you are, pick up H4!
From a competitive aspect, halo reach by far. Anyone who says other wise doesn't know what they're talking about. Halo 4 isn't even halo, it has no descope, no skillgap because it's so easy, and overall is a really shitty and boring game.