For the record, Van Cleef would do the exact same thing he does to everyone he fights-- call his opponent a lapdog, summon adds around 50%, and then die horribly. I'm calling Arthas on this one for sure. Better question: Ony vs. Hakkar (with aspects or without?). Or Vael vs. Nef (as a legendary forum thread, now lost to history in the old forums, once postulated). Or C'thun vs. Kel Thuzad! I'm going to have to go with C'thun for sure-- KT is a lich, but C'thun is an Old God, man!
So, so nerdy. And yet, if you're into the lore at all, so very intriguing. Especially with the Caverns of Time upcoming, expect to see lots more of this stuff. When you're able to get up close and personal with some of the biggest figures in the Warcraft universe, you can't help but imagine what these showdowns would be like.