I have an opinion on the Pokémon anime that the majority of fans would consider to be "unpopular": The show really went downhill and wasn't worth watching once Misty left. She had been traveling with Ash through three regions (two of which were in the main series games); Kanto, Orange Islands; and Johto. Initially, she followed Ash to get him to pay for destroying her bike, which she reminded him about often, especially in the earlier part of the Original Series. I also remember her fiery temper that she often had throughout her time with Ash.
Another "unpopular" opinion (at least on here) of mine is that Serena looks cuter post-haircut and outfit change. Her original look was cute enough, and I had mixed feelings when I first found out about her new look, but it soon grew on me.