My roomates and I played through Half-life 1 in co-op not too long ago, it was pretty awesome I have to say. That game has great trolling potential, almost as much as Doom or Unreal.
Speaking of, am I the only huge Unreal fan here? It's rare for me to run into anyone that thinks the first Unreal game was as awesome as I did. Seriously, for the time (when Quake 2 was just out) it was one of the best, most advanced shooters ever, and it came from a couple of guys working in an apartment that managed to out-do fucking id Software in the technology department (and they still do to this day
).
It's just as much about adventuring/exploring and beautiful scenery as it is about uber-intense shooting mechanics (later shown off in Unreal tournament series). Even now the scenery is fucking great, it's such a unique shooter, it was leagues beyond Quake 2 in almost every aspect, except it had buggy network code on initial release. Somehow a lot of people overlooked it, and then Half-life came and it got buried. Thankfully it was impressive enough and recieved well enough that Unreal Tournament came along to preserve the name.
Anyways Unreal 1 is by far one of my favourite games to play co-op. It's fucking hilarious to screw with your friends, and the levels are so massive and the environments varied that it never gets old. You start off in a crash-landed spaceship and end up everywhere from maze-like underground caverns, to mines, to cities in the sky, to ancient temples and sewers and even in space. Not to mention the guns are pretty imaginitive. Most the guns form Unreal Tournament are present for you to kill legions of native creatures, fauna and monsters, and of course the Alien forces invading the planet it all takes place on.
I personally think it's aged much better than most games, but it's hard to say if a new player would see the greatness it once represented back in the day. Anywho. Any other players?