Either Bayleef or Bulbasaur. Bulbasaur because his personality shined through the most, especially in his leaving episode and the Chronicles episodes. Bayleef because she was one of the few early Pokemon Ash had the actually evolved, and she was adorable throughout but still had that tough streak within her like when she battled Chuck (she didn't hold back with those body slams either!)

I've voted Bayleef, because I feel she should've been given a shot when Ash was in the Battle Frontier.