XCom is crazy hard.  You think you're doing well:  you've got a few dudes leveling up, maybe you've captured an alien, you've got some manufacturing set up.  Then suddenly, san deago falls under siege from the aliens.  You're pretty boss, so you go try and lift the invasion - Wrong move!  As soon as you get off the boat, all your dudes get blown up.  Damn.  You've got some new recruits on the way, but you've lost all your funding and the world gets taken over.

XCom takes the best parts from 3 genres (4X, tbs, rpg) and mixes them up until you can't tell which is which.  The music is great, the graphics are bad and it's so immersive I've lost days playing.