I can always find a quick match in Halo 2--as a matter-of-fact, I prefer its' matchmaking times to Halo 3, where there can be 200,000 people online-or-what-not and I'll sit for up to 8 minutes for them to find a match for me.
I used to play Far Cry Instincts and Predator online about a year ago and sometimes would get lucky enough to have 16-player matches. I always seem to find people online in Conker, but they're fairly bratty. Battlefronts 1 and 2 are spotty, Doom is a ghost town.