Which of Ash's Pokemon faints the most?
I'm not going to bother checking every single episode, since I have better things to do with my life.

However, I can say with a 90% certainty that it's Pikachu. Not because it's weak, but because he has stayed with Ash the whole time, much more longer than any other of Ash's pokemon. So he's battled more and likely to have lost more.