Builder: Not sure who the most well-known is, but barnabas, bushido, king leo, Phantom Knight, and Mr Nobody are the people I think I'd put in my top 5. And apparently brickitect is also well known since he was suggested a couple times, although for some reason I can't think of anything he made
Server hoster: Server quality is meh but let's be honest, Tezuni is probably the most well known. Pecon7 and I think his name is cat123 or something, would follow behind
Forum Member: Depends on what forum you read and how long you've read it. As lame as it is, I would go with Badspot, though technically he could fit as the winner for all of these categories.. lol
Video maker: TurnipBlog I'd say Filipe. Eksi might be of a bit more skilled, but he posts less consistently and has 1/4 the subscribers, which speaks for itself
Fighter: I'd say Lalam or Kalphiter, but Bushido is also well known for that
Eventer: Icygrammer