Powergamer
"Powergamer" is a term describing players who attempt to make their characters in RPGs as effective as possible within the game mechanics, possibly (but not necessarily) by exploiting broken or badly worded rules. It usually used pejoratively, suggesting that the player in question only cares about the mechanical power of their character and this comes at the expense of roleplaying, although many contend that powergaming and good roleplaying are not mutually exclusive endeavours. A somewhat milder term with a similar meaning is "optimiser", which generally lacks the negative connotations of powergamer. At the other end of the scale is "munchkin", which is almost always used to refer to someone who really doesn't care about roleplay and only mechanical power, as is "hilariously" demonstrated by the "Munchkin" series of card games by Steve Jackson Games.
In Summary
Powergamers are total fags. Admit it, if you've played against one in any game, was it fun? Fuck no. Nothing is worse than some little bitch who spams Failbaddon AND Kharn AND three fucking Land Raiders because of a technicality. They're all dedicated transports? FUCK THAT NOISE, GTFO OUT OF MY STORE.