Deus Ex. First play vanilla. Second play without killing anyone except the 3 people you pretty much have to*. Third play only kill when it's "cool" to do so, like some kind of righteous action hero. Fourth play only when my life or the life of an innocent is endangered. Fifth play as a ninja, using only melee weapons, throwing knives and mini-crossbow... and grenades. I like to plan which bio-mods I install so I can have both cloak and radar transparency

Next time I want to do it completely unnoticed, without even rendering anyone unconscious.
Stalker. I get to Chernobyl and then just want to play through again because it's that good, I can't bring myself to even attempt to complete the game because then it'd be over
Also Portal.
* yeah I know there are ways around some of them, but I don't like to run away from a fight
