DP Ash vs XY Ash

XY Ash by far. His strength is shown a lot better in this arc than in any other arc, mainly thanks to 1. a really solid Kalos team and 2. Ash-Greninja. Ash did have great battles in the form of Paul and (ignoring his legendaries) Tobias, but Ash had a much better battle against Sawyer and Alain.

Ash finally beating Paul was a great 3-episode battle, however it didn't change Paul's opinion on Ash, as in the Japanese dub, he already let go of the hate that he had on Ash back in a previous battle and already respected him more. Ash vs Sawyer, however, may seem like it didn't have too much effect, unless you remember that it's kinda like a battle between student and master, a less experienced trainer vs a really experienced trainer. Sawyer losing made him realize he still had room for improvement, unlike Paul, who just walks away with even more respect towards Ash (nothing much really).

Ash had no cheap losses (like Tobias) in Kalos, since he and Alain both had strong Pokemon and an incredibly strong "ace" (Alain has Mega Charizard X and Ash has Ash-Greninja), not to mention this is the first time Ash made it to the finals and, in the process, defeating Sawyer's Mega Sceptile which had a type advantage over Greninja. For Tobias, he brought in a team of pretty OP legendaries, which obviously Ash couldn't handle that well (ignoring his Battle Frontier Battles), making his loss seem really cheap, especially since Tobias's character has not been explained or look into in depth. Alain has a goal in winning the league, which is to prove that he is a strong trainer to Lysandre (and, in the process, also unknowingly complying with Lysandre's ultimate plan). Even though Ash lost it's still justified as we know Alain has trained to be a really strong trainer, and actually deserves to win, unlike Tobias.

And that's not to say how Ash was the entire arc. In DP, he still traveled with Brock, who in a way was still his mentor. In XY, however, he was looked up to by everyone, which helped him become stronger and have more prestige. His relationship with Greninja is scrutinized, too, and how Battle Bond finally activated correctly shows that Ash and Greninja were truly strong together. Don't forget the boss battles, too. I felt like placing the villains arc after the league was a bright choice, as now Ash can show that, yes he may not have won, but he still is an insanely strong trainer. XYZ showed that he almost reached his goal to be a Pokemon Master (sadly, it seems like SM dropped that ideaology)
I feel like other factors that made him better than DP was the artstyle and that it was such a huge jumpback, but imo XY Ash is better than DP Ash