If you're looking for more violent stuff to go along with Battle Royale, check out Blade of the Immortal and Berserk (both published by Dark Horse). They're also both much, much better than BR, more plot, better characters and less melodrama
Action wise, try out the original Battle Angel Alita (Gunnm in Japan) from Viz. It's currently being reprinted now in a cheaper, smaller format. There's also a sequel that changes the events of the originals ending being published now called Last Order but it's not nearly as good as the original.
If you're into cars, guns and over the top "realistic" action, check out Gunsmith Cats from Dark Horse. It can get a little fanservice heavy at times, but if you're enjoying BR it shouldn't be an issue.
For some young action/comedy fun, Viz's Shonen Jump line has some great low priced titles. Try Naruto , Bleach and of course the king of all manga, One Piece .
The Akira manga should also be on the top of everyone's list. It's easily one of the best things I've ever read (not just including manga). It's a little costly, but well worth it.
Raijin also published some excellent titles (Slam Dunk , City Hunter , Fist of the Blue Sky and Fist of the North Star ), however they ran into some issues and it's unknown if they'll continue to publish their books. Slam Dunk and City Hunter are must reads though.
And if you're looking for something a little different and gritty, try out Black & White and Ashen Victor, both also from Viz.