Ash's Worst and Weakest Pokemon

Original series

 * Pidgeotto - Only won one gym battle against Misty's Starmie.
 * Tauros - He should have traded the other 29 Tauros away in exchange for various Pokemon. Tauros was never seen in action until the Orange Islands and won only once.
 * Totodile - Should have evolved, missed opportunity. Lost more times than it has won.

Advanced
Torkoal - Lost so many times. It once lost to Team Rocket and was captured, it was taken down by Norman’s Vigoroth and even got knocked out in a couple of practice matches between Ash’s other Pokemon.

Diamond and Pearl
Torterra - It kept losing battles as it evolved. It should have stayed as a Grotle, or even a Turtwig. Paul had Torterra; why did Ash need one?

Black and White

 * Paplitoad - Super pointless. Ash already had a Water type and would later catch a Ground type. He also lost more times than he won. He even lost to Emolga and Zebstrika, which, mind you, are Electric types!
 * Unfezant - Lost more times than she has won, including losing to Cameron's Riolu, a Fighting-type pre-evolution.
 * Snivy - Lost so many times. Trip had Snivy; why did Ash need one?
 * Oshawott - Stupid personality. Should have evolved, missed opportunity. It was weak on the battling side.

X and Y

 * Noivern - Once he evolved, he lost to Alain's Metang, Sawyer's Clawitzer, Alain's Weavile in the league, tied with Sawyer's Salamence in the league, and pretty much got taken down by Lysandre's Shiny Mega Gyarados. And he didn't win any battle, like at all.

As for Sun and Moon, I currently have none.