While I'm a great admirer of Kirby's Fourth World and its awesome villain Darkseid, I have to relent and say that without a doubt Kirby's greatest villain Doctor Doom. While he created villains which were more powerful and villains which were more depraved, never did he (ably assisted by Stan "The Man" Lee) make a bad guy who combined all of those vile traits with just enough of a dash of nobility to make it all very weird. Rooting for the villain is not how it's supposed to work, but sometimes with Doctor Doom it's hard not to try to understand his point of view, a demented as it can be.
Here are (as far as I can tell) all of the covers Kirby ever produced featuring Doctor Doom. (Would love to find out I"m wrong.) I command you to enjoy!