Conflict: Global Terror (or Global Storm as it was changed to for whatever reason), the last title in the series that was still good, as Denied Ops was a pile of Dung.
Seriously, though, the game was mostly mediocre on the whole but it was the Co-op that stood out. Back before Co-op was prevalent, Global Terror implemented possibly the best Co-op I've seen so far. My wife and I must have gone through the campaign at least a dozen times, which is more time than we've spent on any other Co-op game since. The things that made it so great were, in no particular order:
1. The different characters had distinct roles so you actually had to work together to do well. This is the only real problem I had with Left 4 Dead; everyone would grab the same weapons and the game degenerates into who can kill the most zombies. In GT if everyone tries to fill the same role you're either going to end up picked off by snipers or getting flanked and cut down.
2. Stat tracking. As you progressed through the campaign you earned points for different things (Headshots, bonus objectives, stealth kills, etc.) and these points accumulated to earn you promotions for the character the performed the action. In single player this feature is cool, but not that spectacular. In Co-op it adds a bit of competition to the experience, trying to see who can rank up the fastest, who can get the highest headshot to kill ratio and so on. This feature, combined with the need to work together allows the game to be both cooperative and competitive at the same time.
3. Co-op feels natural. Left 4 Dead and Gears of War did this too, but not many others have. This means that not only does the co-op mode not feel show-horned in, but it goes so far as to seem more natural than single-player, as if the game was designed specifically for it.