tough... can't choose just one... uhh...
there's the Mugen v. Jin fight on the first episode of Samurai Champloo
the two Spike v. Vicious fights ("Ballad of Fallen Angels" and "The Real Folk Blues pt. 2") and the scene where Spike goes crazy in "Jupiter Jazz pt. 1" [Cowboy Bebop]
the Ed v. Greed fights (especially the last one) [FmA]
yeah.... I tried not to make a stupid list, but oh well...
these are the ones that just popped in my head
kickass action sequences!