Battlefield: Bad Company 2. while it's not underrated by any means. it's my personal favorite multiplayer game of this gen. Amazing gunplay and incredibly fun if you're playing with a squad of friends.
Killzone 2. It's as fun as bad company 2 but I thought that some of the maps are poorly balanced and designed. Radec academy with 32 players is an absolute clusterfuck but it's still a great multiplayer game.
Frontlines: Fuel of war on the 360 had amazing multiplayer despite having average graphics, it's a good mix of bad company 2 and killzone 2.
Uncharted 2 online was actually a pleasant surprise for me, I really liked how it flawlessly mixed 3rd person action with platforming. I actually find it better than the single player by a small margin.
Assassin's Creed Brotherhood online is freakin' awesome. I thought Ubisoft would've slapped together a generic deathmatch mode and call it a day but surprisngly it works really well. stalking your targets and then aerial assassinating them never gets old!
