1) COD4
2) MW2
3) Uncharted 3
4) Killzone 2
5) Fifa 10

60% of my total gaming time must have gone into COD4 and MW2

1. Pokemon Diamond (DS)

2. Call of duty MW2

3. Call Of Duty MW3

4. Call of duty 4

5. Call of duty waw

I played a lot of good online multiplayers other than cod but basically your experience is amplified by doing well. so the fun I had in Halo, street fighter, gears of war series, ghost recon, gta.......etc paled in comparison because I was mediocre on the playing field.

Wining an hour long pokemon battle, getting a nuke or just a nice free for all game in cod 4 was enough to make me feel good at work all day if I played right before leaving.

Including PC?

1. Halo 3

My most favourite online multiplayer of all-time

2. Halo 4

Had some problems to get into it the first weeks but I play it daily and love it right now. I think the "big" differences to older Halo games was also why some other people didn't get into it. If they would have played it more I think they would also like it now.

5. Halo Reach

Was fun but wasn't as much fun as Halo 3. Liked the small maps more as the small Halo 4 maps but I still prefer Halo 4 right now. 

4. Battlefield 3 on PC

played around 150 hours which is not much compared to Halo but much compared to other online games for me and played pretty much only conquest. Not sure if I even touched Deathmatch. But from one to the next day I lost the interest, something which never happens with Halo games for me.

5. Ghost Recon: Advanced Warfighter 2

If for co-op campaign, co-op missions or multiplayer against others, I liked all of the online modes. And I was one of the best in the worldwide leaderboards which was also cool because these leaderboards didn't only count the most kills but how you played against other people around you in the leaderboards.

I had the most fun playing Metal Gear Online.. I clocked in over 2000 hours playing it..its offline now tho. :(

Waiting for the new one for MGSV!

1) Mw2
2) Brawl
3) GTA V
4) Mario Kart Wii
5) Starcraft 2

MK Wii
Cod BO
SSBB
...
...

I really don't play that many games online :D

Not in any specific order:

Starcraft II
CoD4
Metroid Prime Hunters(before the hackers)
Pokémon Heart Gold/Soul Silver
Halo 3

Twisted Metal
Super Smash Bros Brawl
Demon's Souls
Army of Two the 40th Day
Resident Evil 5

1-MGO
2-GTA ( IV and V)
3-COD 4
4-UC (2 and 3)
5-RDR

Dark Souls
Monster Hunter Tri
Halo 3
COD4