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XY's my vote for best, but as someone else mentioned, being "best" Pokémon is getting gold at the Special Olympics, it's still a flawed show. I will say Ash v Paul at the Sinnoh League is still the best battle, though, because it has so much more gravity than Ash vs some kid who admittedly looked up to and surpassed bom but still couldn't be bothered to show up until the opening changed to Getter Ban Ban.
SM is just lazy and boring, it feels like. It's easier to animate and it feels like they stopped trying because of that. But take that for the opinion of someone who watched tyne first 10 and was bored to tears; maybe it got better.