As a player I tend toward urpriest's theorist

As a gm I would be an improviser, except that I will create the world in great detail from a political, cultural, and geographical standpoint long before the game begins. Then I proceed to make up everything else. Or I read a monster entry and go with it. Which leads me to another couple gm types:

High roller: Loves tables to roll against, random encounters tend to be really random, since he is rolling on that random encounter chart, expect inadvertent tpk due to the fact that those tables care not a wit what level you are.

MM opener: Similar to above, but his table is the monster manual, and how he opens the book is the die roll. Less likely to TPK since he can turn the page if he gets something out of left field. This how you end up fighting seacats and why occasionally a paladin will end up being attacked by archons.