I love the dagger/assassin like ones in general that have just a small stripe.
It shows them doing a focussed attack rather that just throwing power around
Edit: Like Kaze
Btw, I posted that other thread. Was quite fun
Another spot definitely goes to the art by Daisuke Izuka. I find the stream-like effect really satisfying, simple, and shows how they're swinging their weapon.
Look at Black Knight's, Zelgius's, and Hardin's special arts, though.
I really like Yamada Akihiro's special arts. They're drawn from the ground up rather than just edited versions of the standard attack art. Jagen and Gunter I like in particular.