You'll have to excuse me for mentioning something I only mentioned a few days ago, but since you ask, I have to cast my vote for Reach. The main reason being, I can have glorious fun staying aloft with a jet pack in low gravity, blasting enemies with a concussion rifle (and there's also the fuel rod gun, and other weapons of course). I've had so much fun with that; it's a whole new dimension of combat. I'll refrain from plugging the relevant pages of overenthusiastic babbling on my site, but they're there. For me it's absolutely the best thing about Reach.

Ii don't think H4 offers any aerial firefighting (Special Ops) with a jet pack does it? I didn't have long with it before my 360 bust, but I think the only jet packing I did was in campaign, where the boost seemed frustratingly brief.