Ive said it before, and Ill say it again. If you havent watched Trigun, do so.

I'd also recommend Perfect Blue, its very mature and pretty disturbing, but the story is one that will cling onto you for a long time. Just when you think you know whats going on, BAM ! Everything is then turned upside down. And then later flipped again to the side. Not so much to make everything confusing, you might want to re-watch it as few times though, just to fully appreciate it.

You could watch Akira, its very hit or miss with the movie though. Some love it, some hate it. If you do happen to watch it, make sure to watch the subtitled version, or the second dubbed version. Never watch the original dub, you've been warned.