I've been too poor to do any proper gaming this generation, I just got my 360 last summer, so I've been missing out a lot.
But, based on the 360 backlog I've churned my way through so far, I'd say:
Halo: Reach (the only one of the sequels that actually matched the original)
Super Mario Galaxy (only Super Mario game I actually enjoyed, ever)
Bioshock 2 (Bioshock 1 told the better story, Bioshock 2 ironed out pretty much allt the gameplay kinks)
That's pretty much it for OMGZAWESOME gaming-experiences this generation for me.
Modern Warfare, Gears of War et al were all-right, but nothing earth-shattering.
I'm still waiting for some awesome RPG to knock my socks off, I have high hopes for Xenoblade in August, but I'm trying to keep my expectations low. I guess I'll have to check out The Witcher series too.
Right now I'm playing through Mass Effect 1, and I'm planning on doing the whole trilogy. So far, about 12 hours in, I'm totally enjoying everything but the constant back and forth travelling (I really hope they change the on-world exploration mechanics for ME2, did they?) and load screens.