Vovchanchyn's 36-0-1nc for 96-2000 is still one of the most impressive runs in the history of the sport to me. It is a shame Igor's had to end on a night he had already had two fights while the other guy only had one fight.
Fedor's 32-0-2nc from 2000 to 2009 is up there as the best to me.
Its almost like Igor passed the torch to Fedor, as soon as Igor's dominance ended, Fedor's began.
The GSP, Anderson, Mighty Mouse, Cruz, Wanderlei and Kid Yamamoto runs are the other ones that stand out to me the most off the top of my head.