Honestly, there are just so many to chose from. I like Stone Cold vs. The Rock, but I don't think it was the best. I'm going to say Vince McMahon vs. Stone Cold is a great, classic feud! I don't think I've ever see two people with so much hate for eachother...
On the woman's side, I don't think there is a question: Trish vs. Lita, there was always so much atheticism and passion in their heated rivalry!