The game that started MP fever for me...need I say more?
2. Mechassualt Series
The games that started Xbox Live, and XBL Clan matches. I was lucky enough to be a part of and where I meet many, many friends rivals from.
3. Call of Duty 4: Modern Warfare
The game was one of my most favorable MP games ever.
4. Super Mario Series.
Nintendo saved the console era...must give them credit.
Maybe the only game that you can play on your phone, PC, 360, PS2, PS3, iPod, NES, SNES, N64, Game Boy, keychain...etc