Last night I decided to figure out the win/loss ratio of Ash's B/W Pokemon. I included all non-Team Rocket trainer battles. Though really, every battle Ash has had was either with a gym leader or a rival trainer. So far, everyone he's battled has been in more than one episode, which for me puts them into rival territory. Well, there was an unimportant dude in the Don Battles, but it was still a tournament match, so it counts.
Pikachu and Tranquill have more losses than wins (but Tranquill's only important battle was when it was still a Pidove). Oshawott, Swadloon, and Palpitoad are the only ones with a win record higher than their loss record. Tepig and Snivy are even in wins/losses. Scraggy and Roggenrola haven't been used in any real trainer battles yet.
That said, I was only going from memory and looking through the episode pictures on Serebii, so there may have been one or two battles I neglected to count.
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