Categories:
Builder - honestly hard to say, there's a lot of people who build beautiful things in this game, brickitect has posted lots of houses recently on the gallery which would class him as the "most well-known" for the moment.
Server Host - I'd say tezuni for now because of multiple servers he has going on, that doesn't mean the quality of his servers is any good though.
Forum Member - I don't really know who is "the most well known" but when I think of people who post on the forum i always remember pass, mr.noßody, kalphiter, port and ephi.
Video maker - probably flipe or eksi.
Fighter - bushido? I don't really know of many people who are super fantastic at combat on blockland.