In EU for me it has to be between Posh, Pride or Crumpet (all had great leading skills and made an amazing community).
Posh - Made a very strong community in and outside of Mount & Blade out of players that were not exactly famous when they joined.
Pride - Lead one of the longest standing 'top' regiments, again cool guy with a good community, shame he's Irish.
Crumpet - Lead lots of regiments, some successful, some not but again made a great community.
For NA my favorites were Tico and Alexander but I was less involved in NA so my opinion probably isn't the best.