sci fi... i googled sci fi anime and that's what i found... downloaded some others as well...
cowboy bebop i'll probably watch after this.
Steins;Gate. Watch it right now after Death Note (Please stop at episode 26...). Also I can highly recommend Patema Inverted (it's a movie though, not a series)
Stay away from One Piece, Naruto, Bleach and Hunter x Hunter. They're all mostly fillers.
I'd suggest maybe try 宇宙戦艦ヤマト2199 (Uchuu Senkan Yamato 2199 - lit. "Space Battleship Yamato: 2199") as a really well done short form sci-fi that was recently reanimated. Actually a very pretty Anime...
Then maybe try something more Death Note like such as Code Geass (Code Geass: Hangyaku no Lelouch and Code Geass: Hangyaku no Lelouch R2). It's also a fairly tame mecha.
Then something a bit more action based like Samurai Champloo (one of my all time favourites)
If you dont mind the mecha and enjoy Space Operas like Battlestar Galactica, definately check out シドニアの騎士 (Shidonia no Kishi - lit. "Knights of Sidonia")
If you liked the action of Samurai Champloo, try go something a bit darker like Claymore
If you want something really good but requires a substantial effort to pay attention, 刀語 (Katanagatari - lit. "Sword story") is really awesome.
Then you get "normal" dystopic ones like 進撃の巨人 (Shengeki no kyojin - lit. "Attack on/of Titan(s)") etc.
Of course all this coming out of Japan for each "good/normal" anime, you get at least five "lewd" animes. Some are really good such as キルラキル ("Kill la Kill"), but most of them are junk. Ratings on animedb and myanimelist are normally a good indication whether or not it is worth to endure all the cringing.