Super Meat Boy. Get it, now.

Battlefield 1942 is probably the best online shooter on the console in my opinion.
Castle Crashers is an absolute gem. Such a charming and funny beat 'em up.

I've also wasted far too much time on Geometry Wars :|

Eww.  I'm a Castlevania fan and I disagree.

--

Digital titles.  Rarely you get a great game.  Usually you get what you pay for. 

Comic Jumper
The Maw
Insanely twisted Shadow Planet
Bastion

Those are some of the great XBLA games that came to mind

How can you not want to play Castlevania with your friends. I put over 80 hours into that game. 

SUPER MEAT BOY!!!

Taking notes, you guys play those games...
Well, I barely read those games mentioned around here, I assumed I was one of the few here that play Arcade games.
They're also a reason for me to be speechless when they say MS doesn't support 360. MS Studios has launched dozens (more) of games from 2010 till now.

ilomilo
Quarrel

Very underrated games (by gamers, not by reviewers).

There's a lot lol. My personal top 10 would be Bastion, Walking Dead, Trials Evolution, Shadow Complex, ilomilo, Castle Crashers, Beyond Good&Evil, Dust, Limbo, A World of Kelfing(Was free). Btw, check out Ascend; F2P game that's pretty good.

Most of these games I bought when they were on sale at one point, like I do with most xbla games, but I would have paid full in a heart beat for any of those

It's actually Battlefield 1943. :P

Also, giving a nod towards Defense Grid.