If you just want good coop gaming sessions, go with Heroes Unleashed, a massive mod which pulls together just about every custom map and weapon out there. You'll have loads of different terrains to play on.
If you want well-scripted campaigns with custom maps, weapons, kit and missions, I'd recommend (in this order):
Frostbite
Year of the Monkey
Rockall*
P2
* Main focus is on coop gameplay.
That will probably be enough to last you for a long time, but other mods that I can recommend for coop from personal experience include:
9ms
Tactical Gamer 3.0
Project Sentry**
H-Hour's Server-side Mission Pack**
8 Days and 12 Weeks (the original coop mission packs -- stock maps only)
** These were created by me, so take the recommendation with that in mind.
There are lots more, but these are the ones I have used myself in coop gaming sessions over the years.
If you're pulling in people who are new to Ghost Recon or who may be very rusty, I'd recommend starting with Frostbite, Year of the Monkey or H-Hour's Server-side Mission Pack, as many of these missions include lower enemy counts that are more reasonable for small or out-of-practice teams. If you want tough inserts, try Rockall, P2, 9ms or Tactical Gamer 3.0, all of which were created for experienced coop teams.