Interesting that someone said Mark Todd, the first thought that popped into my head when I saw this thread was "definitely NOT Mark Todd". Don't get me wrong I could not ride the horses he does as well as he does but he's a very physical rider and he always looks slightly hunched over and manic- very flappy elbows

The riders that stand out for style for me, Laura Renwick and Ben Meyer in SJ, Mary King and Lucinda Green both very stylish in eventing and Charlotte Dujardin is a beautiful rider.
A good counter thread would be "Who is the least stylish rider but gets the job done just as well?"
I remember being told by a PC instructor that Will Funnell "rides like a farmer but his horses seem to like it"