Eh, I'll probably agree on Palpitoad, performance wise that is. It's only really been used by Ash three times, but has won each match. I'd say it is probably Ash's strongest Pokémon at the moment, which is probably why we're not seeing as much of it right now.
Other than Palpitoad, I'd probably say one of Ash's starters, which are pretty much being spammed. Snivy has proven herself to be a good in battle, but lacks development. Oshawott, although receiving the most screen time, it hasn't been doing too well lately, losing to Chili's Pansear in one of the more recent episodes, and losing to Bel's father. Although Tepig has had quite the rocky start, it has been been quite useful to Ash lately, putting up a fight against Ingo and Emmet and taking out Koharu's Mandibuzz in one of the recent episodes.