Here you go, my top 10. Its in no particular order :
Ash Vs Clemont(Kalos Gym Battle) Really impressive animation, tonnes of build-up, awesome teams. Shit was cash.
Ash Vs Paul(Sinnoh League) While it drags on for a little too long the battle's pretty freaking cool. Probably Ash's best roster ever in the show. My second favourite of Ash's Pokemon, Infernape (my avatar is my fav), wrecks house.
Ash Vs Gary(Johto League) Ash uses some interesting Pokemon (Pikachu doesn't even battle). Gary's f***ing Gary. Scizor's dope.
Ash Vs Korrina(Kalos Gym Battle) Really well animated (all of the XY Gym Battles are stunning. Especially compared to Gens 1-5). Mega Lucario's awesome. Ash's team's pretty dope (Hawlucha, Pikachu, Fletchinder)
Ash Vs Drake(Orange League) The first 6v6 battle in the show. My second favourite dragon Pokemon Dragonite kicks some ass.
Ash Vs Noland(Battle Frontier's Battle Factory) Charizard Vs Articuno.Cool as f***.
Ash Vs Brandon(3rd Battle, Battle Frontier's Battle Pyramid.) Ash battles to get his final Frontier Symbol. Pikachu Vs Regice, man!
Ash Vs Tobias(Lilly of Valley conference) Seeing Ash taking on a Latios and a Darkrai is so awesome.
Ash Vs Harrison(Johto League) Both battlers have cool teams. Climaxes in Charizard Vs Blaziken (not that I liked the climax). First look at some Gen III Pokemon in the series which made it really dope at the time.
Ash Vs Maylene(Sinnoh Gym Battle) Buizel goes up against Lucario. Aura Sphere Vs Water Pulse. Well animated.Cool use of abilities. Fun to watch.