Charizard is probably Ash's strongest Pokemon, especially after his valley training. After that I'd go Snorlax and Sceptile. I'd like to see him with a fully evolved water starter, so perhaps an eventual Samurott or Blastoise. Krookodile or Gliscor. And of course Pikachu!
So my ultimate Ash lineup would be...
Pikachu Charizard Sceptile Blastoise Gliscor And Snorlax (sub-Staraptor, Pidgeot, or possibly Krookodile if Snorlax is too sleepy to fight).
There's nothing that sucks worse that that moment in an argument when you realize that you are wrong.