My take...
PURE CO-OPERATIVE GAMES
Space Alert (20 minutes) is the pick of the pure co-operative games. Vlaada wanted to design a game which wasn't basically a solo game (as most pure co-op games boil down to), and succeeded brilliantly. The CD track provides the game timer and tracks when threats occur, but what they are and their speed is determined by cards and a variety of random threat tracks.
This game has a lot of replayability and is wonderfully fun. Highly recommended.
Pandemic (30 minutes) has great mechanics but suffers horribly from the "I'm a solo game" flaw. I personally find it a bit too easy to solve as well. Yes, we'll lose games, but that's due to horrible draws rather than bad play.
Lord of the Rings (1 hour) is a game I rather enjoy. It's moderately long (hour+), but is surprisingly replayable and doesn't suffer so much from the obvious strategy that leads to solo play in Pandemic.
Arkham Horror (2-3 hours with 4 players. Might be shorter, might be longer) is big, sprawling and, for inexperienced players, will take entirely too long. I've had a lot of fun this as a solo game, but I think it handles up to 4 players very well (5-8 might require you to set up for a whole evening). What AH does well is provide theme and variety. There is a certain amount of strategy that can become utterly derailed by bad luck.
TRAITOR CO-OP GAMES
Battlestar Galactica (3 hours with 6 players) is the best I've played. It has good mechanics to begin with, but its the confusion over which players are Cylons that really makes the game work. You know how many Cylons there'll be, but there might not be any until halfway through the game!
Shadows over Camelot (1 hour) is nice - I've had some good games - but the traitor element really doesn't work as it should. While in BSG, Cylons are more effective unrevealed but can still have fun when revealed, the traitor in Shadows is utterly boring once revealed.
Betrayal at the House on the Hill (30 mins-1 hour) is awesome fun, but very, very random. Some games will come down to the wire, others peter out into dullness. It's very clever - and I'd definitely look out for the reprint - but be aware that not every game will work.
Cheers!