PS3:Gran Turismo 5&6 - Playing GT5 multiplayer in a simpit is still the best car sim experience I have had. Sadly the guy I knew that had that setup moved before GT6 dropped.
360:Halo Reach - This was the best Halo since the first for me. Beat the campaign probably half a dozen times in coop.
Zoids Assault - Sure it is not a great game and it is stupidly difficult. But it is Zoids as a SRPG. When I sold my 360, it is the only game I kept.
Wii:Wii Sports - I have a Wii hooked up for Wii bowling this weekend
Lets Tap - One of the best drunk party games ever created
Kororinpa 1&2 - Excellent tilt puzzle games
Mario Galaxy - The sequel to Mario 64
Monster Hunter 3 - I spent an undisclosed large amount of time playing this online with friends. I was still terrible at it, but had a ball trying to help others.
Tatsunoko vs Capcom - No other game let me play as Jun and ChunLi. Well I guess Mugen...Still gets played in regular rotation at parties, partially since it is a 4 button fighter.
Disaster: Day of Crisis - The Monolith game to buy. Solid Action Adventure disaster game. I expected it to be nothing but QTEs, but it was a surprising mix of genres.
Multiple Platforms:Castle Crashers - I rank this in the top 5 beatemups of all time
Battle Block Theater - Easily one of the best coop platformers ever made, though I am a huge fan of the game that it is based off of; PDA Game, or as we call it TV Head game
Final Fantasy Crystal Chronicles: Echoes of Time - This is a Wii/DS title with online crossplatform play. The best ARPG of the generation. Gameplay is fast and simple, but it sucks you in with customization and combos.
Bioshock - If for no other reason, walking around and looking at Rapture.
Portal 2 - Exquisite single player, stellar coop. I am still trying to make a combustible lemon.
Borderlands SeriesLeft 4 Dead 2 - A perfect coop FPS. I do not say that lightly. I have well over 1000 hours in the game (mostly as Ellis and Zoey) and a couple weeks ago I heard an Ellis quote I had never heard before. Heck I even played this one with DSH a couple times:
xen is a buddy from college that I have gamed with weekly for fifteenish years now, same as Ack.