Cowboy Bebop is the go-to for just plain fun science fiction anime. It has a little bit of everything - space bounty hunters, buddy-cop, Star Warsy space battles, con games, spoofs, film noir... Lots of good humor and a GREAT jazz score by Yoko Kanno (as does GitS). Bebop is the anime that got a lot of Americans hooked on anime.
Planetes has been compared to 2001 in its near-future-space styling and story. A very good story about average folks who just happen to work in orbit, collecting space junk, and their hopes and dreams.
Definitely those two, love them both.
I also recommend:
, it's about a japanese surgeon living in Germany during the 80s and 90s, he saves the live of a young boy who turns out to be quite dangerous.
, that's about a guy who works as a bodyguard in Tokyo, he only works for beautiful women he lusts after. After the death of his parter the partner's younger sister takes his place. The series is a mix of action and slapstick humour with a bit of romance between the two main characters Ryo and Kaori.