Which gamer that you have played with has most affected the way that you play?
I'm gonna cheat here, because I only played with this person twice, but his impact on my gaming was incalculable. That person would be Aaron Allston. I was lucky enough to GM a couple of Champions games he played in back in '92. We had fun, and he was a blast to have at the table.
Of course, his impact on me came from his work as a game designer. His Strike Force is still the Gold Standard for showing how to run a supers game. Of course, his influence extends beyond Champions. He wrote "The Grand Duchy of Karameikos" for D&D's Mystara, one of my favorite adventure settings. His "Dungeon Master's Design Kit" is a brilliant (and underrated) step-by-step guide to creating adventures and campaigns. He also compiled the D&D Rules Compendium, no mean feat.
So, a bit of a cheat, but an honest one.
Honorable Mention goes to HERO guy, L. Douglas Garrett, who ran the "Justice, Inc." game that showed me how to run games in a cinematic fashion.