Resident Evil 5. It's the best one in recent years at least.

Metal Slug games on NeoGeo, Rampage series, WWE/WWE 2K games and Killzone 3.

LOTR - Return of the king PS2 cant beat it

Since many people already said the ones I was gonna say, I gonna pick something a bit different

Mortal Kombat Shaolin Monks for the ps2! I had a blast with that game with my friends, just killing everyone and doing crazy combos and fatalities! Just awghh

Resistance 2 coop
Uncharted
Takedown: Red Sabre

Call of Duty MW2. those spec ops were awesome

Non-FPS:

Conker's Bad Fur Day - Heist mode

Kameo: Elements of Power

Super Mario 3D World

Super Mario Galaxy 2

FPS:

Halo: Combat Evolved

Perfect Dark

Perfect Dark Zero

Left 4 Dead 2

After some contemplation, I've bolded the winners of each category.

Seiken Densetsu 3, Secret of Mana, Shadow over Mystara, Snow Bros., Super Pang, Battle Circuit, Midnight Wanderers, Gunstar Heroes...

Resident Evil 5, that game was a blast to play with friends. Actually playing with a friend was pretty much the only way to enjoy it.

It's a close one between Contra with my brother on NES or Super Mario 3-D World with my wife and kids on WiiU. I give Contra only the slightest edge. The first Halo was really great for a FPS co-op experience when it first released. To my surprise, for all of it's bugs and odd choices, post patched Aliens: Colonial Marines was pretty fun with my 6 year old son(though it pales in comparison to the ones above).