What's yours?
For me I'm not sure if I can really number these because they all are so well done, but here is what mine would consist of:
Naruto vs Sauske (Naruto): I know I'm probably going to get a lot of for this one, but this fight really was well done and well animated. In fact, I would go as far to say that it is probably the single most well animated episode of the entire Naruto series. At least up until the next episode where Sauske decides to transform into a drag queen gargoyle.
Spike vs Vincent (Cowboy Bebop Movie): I hardly remember this movie, but this fight scene still sticks out in my mind, which has to mean something. I think this fight really showed that Spike is deadly with or without a gun.
Asuka vs Eva Series (End of Evangelion): This is hands down the best scene in the entire Evangelion series. Asuka not only defeats an entire battalion, but goes on to hand other Eva Units their asses all under to what I believe was 3 minutes. Unfortunately, this fight doesn't necessarily end well for Asuka, but because of it this fight has one of the more powerful scenes in Evangelion.
Takamura Mamoru vs Brian Hawk (Hajime No Ippo: New Challenger): At first instead of putting this I was going to put Ippo vs Sendo (championship fight), but the animation and quality of this fight is just so well done, it just begs to be mentioned. The build up for this match was just so big and when it finally happened it just pulls you in and never lets go. Everything about this fight is on point and there really isn't much I can say about it outside of just recommending watching it. Oh, and it also has one of the most awesome knock downs in the entire series.
Wrath vs Gluttony (Full Metal Alchemist Movie): A kid with a metal arm and leg fights an overweight tentacle monster that wants to eat everything. Also, his tentacles have faces of his face. What more do you need?