Anime is a media, like Film, and has many genres inside of it from Magical Girl and Sci-Fi and Slice of Life and Drama.
I would records to start with some classics like all of the the Ghibli movies, Evangelion, Cowboy Bebop (just in case you forget
, Goldenboy, Great Teacher Onizuka, Princess Tutu and so on. I also suggest Eve no Jikan, but that's too new to be a classic.
As for other anime with dark themes or in outerspace there is Akira, Black Lagoon, Ga-Rei Zero, Gunslinger Girl S1, Mnemosyne, Mouryou no Hako, Outlaw Star (my guilty pleasure), Pale Cocoon, Serial Experiments Lain.
Almost forgot: Watch Macross, not Robotech.