Depends on the system, really. I'm a big fan of Savage Worlds and ORE, personally, but I know a lot of freeformers aren't fans of dice systems. A really light system like Risus or Wushu Open is generally the most coherent you can get going on a site like this. Actually, if I had to suggest a system for battle manga-type games it'd likely be Wushu, since it's one of the few systems that actively disregard powerlevels and the laws of physics in favor of doing as much cool stuff as possible.

As far as homebrew stuff goes generally I've found a hard-and-fast this-beats-that sort of thing rarely works well and generally runs counter to the "powerlevels are bullshit" nature of SHONEN BATTERU-type things. Homebrew systems that add an element of chance, on the other hand, tend to needlessly bog things down in a forum environment and make players afraid to do really stupid shit, which is something of an issue becuase doing really stupid shit is pretty much what One Piece is built on. That, and I've seen a lot of them end up being horrendously broken in the worst possible ways.

So basically what I'm saying is in a game like this you may be better off starting with an honor system where you trust your players to focus more on making things enjoyable for everyone and doing cool shit instead of being weird freeform tryhards obsessed with winning and ending fights as fast as possible. That's just me, though.

EDIT: Upon scrolling back up I think I misread what you were saying but my point still stands I guess.