Resistance 2 co-op was indeed pretty good. It's the only online multiplayer that had me hooked for a while since HL2 deathmatch. 8-player co-op was great fun. I think Far Cry (3?) also had some online co-op. It was only 4 players though and had some issues.

Yeah individually games like Wolfenstein The New Order, Goldeneye 007 on Wii, and COD World at War/Black Ops II are up there for me, but I find the series they come from be hit and miss.

Halo. Excluding Halo 4 which is honestly the worst AAA game I ever owned in terms of disappointment and budget vs quality, but still Halo because the first three games are masterpieces and had so much content. Halo 5 at least had a fun multiplayer despite a campaign as bad as Halo 4. I also like Quake, COD, Doom, and Half-life. I used to like Battlefield, but I have not liked it in years. I never enjoyed Far Cry, Wolfenstein, or Bioshock despite the fact that I've played multiple games from each one of those series. Wolfenstein and Bioshock are games that I feel like were a waste of time and bored me to death. I will give a mention to Metroid Prime because I liked it miles more than those series. I'm looking forward to a real Halo campaign which 343 at least seems to be attempting with their open world previews. The delay is honestly fine with me and I'm hoping the Halo Infinite rework will make the story less cringe than 4 or 5. I also did not buy a switch because of coronavirus. Now they are in stock, but I'm waiting for the switch pro and I hope that the new Metroid Prime 4 will be a good game and take advantage of that upcoming hardware (either better frame rate or slightly higher resolution with same frame rate). 

A few folks have mentioned Metroid Prime; my reasoning for excluding it from the poll was that (and I know this has been an ongoing debate since 2002) I see it more as a first person adventure game, which is how it's developers describe it.

If I were to count it, it'd be #2 for me behind only Bioshock as all three Prime games are amazing.