Personally, a few come to mine that shifted my taste.
- Lost Planet: this was the first game that really got me into multiplayer shooters and gaming in general. Being able to pilot a mech, navigate the map using grappling hooks, being able to play as Akrids in multiplayer opened up a lot of different ways to play. Also the taunts added plenty of humor to the game.
- Metal Gear Solid 4: arguably the best single player of any game I've ever played, actually got me interested in games with expansive stories. It was a great break from CoD at the time.
- Team Fortress 2 & Gmod: both of these games converted me into a pc gamer. Custom servers were great and the quirkiness of both of them keep me hooked to this day.