My favorite is ichigo versus grimmjow that battle was epic

Ack I can't think of just one!

Goku verse Vegeta (the first one before the series became too ridiculous) Goku verse Frieza was pretty awesome to.

Naruto verse Sasuke in the Valley of the End.

Ichigo verse Grimmjaw

There are others but these are probably my favorites.

Any fight with Alucard from Hellsing lol...

Vash vs. Knives from Trigun...

Jiraiya vs. Pain...

....why can't I think of any more? this is annoying...

Samurai champloo Gin vs. the sun flower samurai(i forget his name)that fight was awesome.

naruto vs sasuke and ichigo vs grimmjow

the fight right now between naruto and pein. so far just amazing and awsome

Gotta agree Alucard sure makes a fight interesting, wonder how hed go against Itachi

I enjoyed Asuma's last fight with the Immortal guy and the Knitting guy, and I liked the fight on top of the hospital the first time it was nearly Rasengan vs Chidori, oh and also when Sakura just absolutly destroys that poison lady when they are finding that ghost or what not

Gluttony Vs Wrath in the FMA movie was pretty groovy as well, and a couple of scars fights with the homunculus, If you consider it a fight the attack on Ishbahl was pretty tough looking, not that I agree with the actions the military made.

To pick a favorite From naruto, Id go with Team7's fight(s) against Haku and Zabuza, and when Zabuza turns around after to take on all those goons with a kunei in his mouth, thats class.

Over all Fave would be Goku V Pikkon

Vegeta vs Goku for the 1st time on earth. This battle was epic.

Goku vs. Freiza was awesome too.

Z Fighters vs Cell at the Cell games.

Vash vs. Knives

Sasuke versus Naruto

Naruto vs. Sasuke (Valley of the End)
Kenshin vs. Shishio
Goku vs. Frieza
Naruto vs. Pain
Kurono vs. Buddha (Gantz)

Totally Ichigo vs. Grimmjow. Any fight between those two actually.

Jiraiya versus Pain
Naruto versus Pain
Son Goku versus Old Piccolo
Son Goku versus Grandpa Gohan
Son Goku versus Kid Buu
Gogeta versus Omega Shenron
Hidan & Kakuzu versus Kakashi & Team 10

Okay this might be long.

Kakuzu and Hidan vs Kakashi, shikamaru, Ino chouji

Deidara vs Sasuke

Nnoitra vs Kenpachi

Mayuri vs Apporo

Sasori vs Sakura and Chiyo

Any other fight that ivolves Akatsuki and Espada =D Either in anime or not....But these are my faves

naruto vs sasuke in shipudden