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. The term is 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 endeavors. 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 the "twink" or "munchkin", which is almost always used to refer to someone who cares only about mechanical power (to the exclusion of roleplaying), or is willing to out right cheat if he thinks he won't get caught, as is 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? Did you like having your balls ground to a pulp? 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 MY STORE.
For anyone playing the game simply to have fun and enjoy a good story, a powergamer is your number one enemy. Avoid at all costs. They'll fuck your shit up six ways to Sunday simply because they can.
*SenZar does not count.