Powergamer
"Powergamer" is a term describing players who are interested in winning and, more to the point, winning big. Exactly how they do this depends on the game.
Powergamers who play RPGs generally attempt to make their characters 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 there is nothing intrinsic to powergaming that inhibits good roleplaying.
In Magic: The Gathering, powergamers are also called Timmy or "Timmies", after the randomly-chosen name that Mark Rosewater assigned to a hypothetical powergamer during R&D. To this day, most MTG sets have at least a few "Timmy cards", massively powerful creatures or spells that will put a severe hurt on the opponent -- as soon as Timmy has enough mana, and actually manages to draw his nuke. The card "Timmy, Power Gamer" was released in the Unglued set, whose rule and flavor text reflected the stereotypical Timmy gameplay style -- draw cards and put down creatures until The Big One comes out, or there are enough monsters to steamroll the opponent.
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 outright cheat if he thinks he won't get caught, or there will be no consequences if caught, as is demonstrated by the "Munchkin" series of card games by Steve Jackson Games.
In Summary
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 that guy who spams Abaddon AND Kharn AND three fucking Land Raiders because of a technicality. They're all dedicated transports? FUCK THAT NOISE, GTFO MY STORE.
Ever played against an IG armoured list. No? Pray you never do, because that list displays a level of asshattery (and disposable income) on a godlike level. Seriously, fucking TANKS as TROOP CHOICES?! OH FUCK THE FUCK OFF!
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.