I'd recommend Cowboy Bebop for sheer coolness. It's not a great story-based anime but the characters and music make it great.
I agree with School Rumble, it had me in stitches so many times. Comedy anime is great, so if you like an anime that doesn't take itself too seriously, go for Samurai Champloo. If you like GTO, there is an earlier story of it called Young GTO! Where Onizuka is like 15. There are 5 OVA's of it out, but it's a bit hard to find.
And if you want very story-driven anime, Code Geass and or/Death Note are both great series.
I personally prefer manga, because it's easier to get and there's so much of it out there.