MGO (from MGS3)

Well, the Halo games (2, 3, and Reach are the ones I've played online) are among my favorite. Definitely my favorite FPS to play online. For co-op, it's definitely Resistance 2's online co-op. But I think the overall winner is... Warhawk. I played that game to death back in the day. The online is a ton of fun. And I'm still waiting for the sequel.

...And if it wasn't for Warhawk, VGC, and real life friends with a PS3, my friends list would be empty. Warhawk actually contributed the most to my friends list. 

On Console

Metal Gear Online(Metal Gear Solid 3 and Metal Solid 4 Versions), Biohazard(RE) 5, Ninja Gaiden Sigma 2, Shirokishi Monogatari: Inishie no Kodo((White Knight Chronicles), Shirokishi Monogatari: Hikari to Yami no Kakusei((White Knight Chronicles 2) 

Uncharted 2, when I heard ND was putting online in, I thought "thats going to suck" when I read reviews and they covered online only in passing I thought "it must really suck" but then I played it and I was sold. Never thought it could be that good. While everyone was playing MW2 I was playing U2. Goldeneye gets an honorable mention. Golden Gun mode alone is worth the price of admission

Phantasy star online on the dreamcast... especially the early years before all the hackers came and destroyed it..

the "word" translate option was just fantastic and you could string together fixed words into sentences and be translated into 5 different languages.

playing with some real hardcore japanese, american and european gamers, and it ran fantastic on 56k dial up modem in those days.  

ate up way too much of my life tho and I was up till daylight playing this game..

Battlefield 2 on PC, AWESOMEST GAME EVAAAAARRR, even though I had mega shitty computer so when I used a plane I was skipping like 10 metres a second, it was still awesome.

COD4 on X360, amazing game for me, I sunk quite alot of hours into it, but never in a competitive way, never tried to get 10th prestige as I think it's a waste of time to go THAT far. Au Contraire, MW2 was the worst online experience for me, even though I sunk even MORE time in that, thanks to friends haha lol

Little Big Planet on PS3. Reasons dont matter for this thread, 

PC : EVE Online

Waging wars against thousands of human players with your alliance on one server is the best gaming experience I had. Getting involved in such wars was awesome !

Console : MH3

Coordination with random players, as a newbie being helped by veterans, then veteran helping newbies... What I like was the cooperation in MH3, you're so realived when you beat a monster.... then the carving fest at the end ^^

Monster Hunter 3 easily for me.  Nothing compares to finally taking down my first Rathalos with 3 complete strangers

The same here.  I'm retired now, but this guy and several others have showed me that the difference between greatness and mediocrity can be a split second decision.