It's between DP & AG,both had a huge impact in the series as a whole.DP has the best battles strategically,the best rival and the best league but I have to give AG the edge because of a few things.First I'll say that AG was the last time I actually cared for Ash as a character,yeah he vastly improved in DP as a trainer but as a character he became bland and lost the charm he had from the previous series.Secondly the contest rivals in AG were a huge plus,I kinda see Drew as the Gary Oak of that series as far as rivals go,him and May both went at it and faced each other in contests both eventually grew to respect each other.
Drew had a little more experience than May so he was a driving force for her improvement as coordinator.You also had Harley who was known for playing mind games with May and throwing her off like when he once joined up with Team Rocket to try an sabotage one of her contest battles.I just didn't get the same feeling with the contest rivals in DP,I thought Zoey was meh as a rival she felt more like a mentor for Dawn,Kenny of course was a flop,the only rival I thought was decent was Ursula.Lastly the Battle Frontier which was a rare occasion for Ash to actually rotate his team,plus the Frontier battles were intense.