5. Half-Life 2 (2004)
Half-Life 2 piled on spindly creatures, foreign weapons, and mechanics previously unseen in gaming -- the gravity gun leaps to mind. Alien face-huggers. Sadly, despite being told for more than a decade that the third game is on its way, few gamers actually believe they'll live long enough to see its release...
4. Bulletstorm (2011)
Unforgivably underrated, Bulletstorm may just be the funnest -- "most fun" doesn't sound fun enough -- game on this list. Using an energy lasso and an arsenal of twisted weapons, players tossed, slammed, decapitated, exploded, nut-shot, or tore enemies in half to drive up a combo counter. Visually gorgeous and a ridiculously addictive combat system rewarded imaginative mayhem.
3. Borderlands 2 (2012)
You're certainly never going to find a more hilarious shooter, or an FPS brimming with more pop-culture Easter eggs, or an action RPG with a weirder set or characters, or a cooler collection of guns, or a better, more handsome villain, than in Borderlands 2. The game's a near masterpiece. Play alone or with a friend, but we dare you not to laugh.