Perfect Dark is choice. It's still a great shooter in it's own right, but I have to admit that I get enough joy from nostalgia alone to want to play this with friends. Playing alone just doesn't give me the same feeling. Still great though.
Halo Reach just got a patch that fixes its issues. That's always a fun co-op if you can stand geographical oddities and the like.
World at War. That has co-op campaign and a good soundtrack. Might be good for a Sunday afternoon play through. Never know.
Black Ops has a "Nazi Zombies" mode you may remember. I spent two hours co-op Dead Ops Arcade with Lady Serana last night. Fun with friends!
Haven't tested online in ages but The Orange Box (Team Fortress 2) is on Xbox One now. Got that. Could be fun.
Gears of War 1&3 I have both via Games with Gold. I never got into the series for its story, but if someone needs a co-op partner, I'm always down for some action.
LEGO Star Wars: The Complete Saga is a really fun co-op if you're a fan of the source material. If not it's just another LEGO game. I'm good to co-op Super Story mode with anyone willing.
Pac-man CE DX+ Willing to compete for high scores with friends. One friend has me beat by a million points so I may never be on top but I will challenge anyone.
Portal 2 is really low on this list for some reason, but yeah. That game is co-op friendly and plays great on XONE. I have yet to finish the single-player part but the co-op can be a good time.... As long as you work well with others.
Perfect Dark Zero because sometimes disappoint reminds you to put down the controller and go play real life in co-op mode.