Berserk, defiantly. Despite a crappy first episode and a slowest start, it’s amazing. Very dark, very mature, very bloody. (Over the top at times) The writing is outstanding as well. Beyond the best fantasy anime out there, (Its only real competition is the also excellent Vision of Escaflowne) it’s arguably the best adaptation of fantasy to screen, ever.
Escaflowne’s a terrific show as well. The beginning’s a little cutesy, but it settles in well.
The second season of Kenshin is considerably darker in tone than the first one. (And if you’ve only seen it on Cartoon Network, you’ve never seen it. It’s being butchered. They should have run it on Adult Swim, there’s just too much blood) The OAVs (which were just re-released in a dirt-cheap 4 episode disk, called Betrayal and Trust) are MUCH darker than that. There are some incredible fight scenes in them as well. I suspect you’d like them a lot. You don’t need to have seen the TV series. (It’s a prequel)
Another dirt cheap set is the oddly titled Eat-Man ‘98. Had a very similar ‘feel’ to Cowboy Bebop. One of those ‘wandering badass’ male fantasy shows that has minimal redeeming social value, but is fun as hell. Might be hard to find if you’re looking to rent, though. Only two of the episodes have a dub track, though, so if you don’t like subtitles, you’ll probably want to pass.
If you’re just in the mood to watch some ass being kicked stylishly, Spriggan’s not a bad choice. Not much else, but it sure is animated well . . .
If you like Lupan, you might want to check out Golden Boy. Almost no action, but hysterical in a similar, slightly blue manner style. A freelance temp wanders around, ‘learning about life’ while engaged in a string of meaningless part-time jobs, invariably surrounded by girls, his hidden brilliance ultimately solving all problems, most of which are caused by his social ineptitude. Sort of the ultimate geek slacker fantasy.
I’d say ‘wait a bit’ on Eva. It’s sort of meant as a wrap-up on the entire giant robot genre, you’ll get more out of it if you’ve seen some of the shows it’s inspired by/borrowing from. Also, after seeing a few series and getting used to how vague and indecisive the endings tend to be, you won’t be so pissed when it refuses to resolve a single important plot thread. 
Speaking of giant robots . . . It’s a bit hard to find these days, but Macross Plus is amazing. It was done by the same team who put out BeBop, Big O, and Escaflowne. The aerial combat scenes, of which there are many, are actually better than the ones in BeBop, believe it or not. There’s two different releases, a four episode OAV and a movie. (The movie has a better ending, but cuts out some important subplots) It’s the ‘true’ sequel to Macross (which formed the core of Robotech) but you don’t have to have seen it.
Let me put it this way. Remember how good everyone hoped the new Star Wars movie was going to be? How it was supposed to keep everything that made the original good, but take into account how the audience had aged and the medium (science fiction, special effects) had grown more sophisticated? Well, Macross Plus actually pulled it off.
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‘We were expecting too much’ my bio-armored butt . . .
