Metal Gear Solid series - it brought gaming to the mainstream.
Gears of War - chunky chainsaw dismemberment! Plus I think it'll turn out that humans are the ones that actually started the war. I love those twist were the "good guys" aren't what they really seem.
Super Robot Wars series - I've been playing that series for a decade now.
Devil May Cry series - Makes you feel like a total badass.
Resident Evil series - Pop goes the zombie head.
Silent Hill series - Creepy and moody.
Fatal Frame series - Downright scary the first time I played it.
GTA series - with a gameplay copied by others more than you can shake a stick at.
Tomb Raider (the newer ones) - it stars a kickass sexy lady. with brains.
Final Fantasy series - the yardstick for which all epic RPGs are measured.
Suikoden 2 - my personal favorite for BEST RPG EVER!!!
What doesn't kill you will still hurt...