For me its gotta be episode 270 "Can't Beat the Heat" because of the classic rivalry of Ash and Gary. This battle is the battle we've been waiting for since we first saw Ash and Gary on episode 1. It's not like the other ash vs gary fights where it ended up one sided. This is the epic conclusion. I just wish the Electivire vs Pikachu battle that took place later in the future didn't happen because that kind of ruined the awesome ending of the two rivals with Gary on his way to become a researcher.