Both Mike and Nick picked Halo and Halo 2 for their favorite co-op experiences for the Xbox. It's unsurprising, as it's hard to think of the Xbox and not think of Halo.
Staff Quotes on Halo and Halo 2:
"Couch play (both competitive and co-op) practically defined the Halo series on Xbox, and probably accounts for about 90% of the total use I got out of that system." - Mike
"They defined that system and are still some of my favorite co-op games of all time." - Nick
Nick Puleo: Halo series, Crackdown, 50 Cent: Blood on the Sand, Gears of War, Gears of War 2, Gears of War 3, Schizoid, Geometry Wars 2, Toy Soldiers: Cold War
Mike Katsufrakis: Gears 1,2 & 3, Halo 3, Halo 3: ODST, Halo Reach, 50 Cent: BitS, Dark Souls, Rock Band 2, Castle Crashers
Marc Allie: Halo Reach
Jason Love: Halo 3
Tally Callahan: Lara Croft and the Guardian of Light, Castle Crashers, Tales of Vesperia
Chris Metz: Left 4 Dead, Left 4 Dead 2, Portal 2
John Bardinelli: Castlevania: Harmony of Despair
The Xbox 360 was the system that, all together, participating staff cited the most favorite co-op experiences. The 360 brought people together online in a way that only the PC had previously succeeded in doing. Certainly a major co-op platform, so it's no wonder that some of our staff members had a hard time narrowing their favorites down to one (or even two!).
Staff Quote on 50 Cent: Blood on the Sand:
"50 Cent was a surprising game, Nick and I loved it so much we named our Game of the Year award in its honor." - Mike
Staff Quote on Halo Reach:
"Of all the Halo games, I think this is the best. The only game in the series I replayed the campaign three times, even on Legendary, in co-op. One of my best gaming experiences ever." - Marc