the best sci-fi is definitely battlestar galactica, if you think anything else, you watch it again! it has best effects, best story, best music (!), best characters, best dialogues... its just the best series ever made. It's dark, violent, with politics, mysteries, myths. Almost nobody is 100% right, everything has two sides... you have to think when you watch. and when you see the 4x10 episode you will stay with your mouse open staring at the screen wishing for more and more and praying for them to launch the prequel Caprica as soon as possible
star trek is great, but come on... everything has a happy-end, its too utopic, maybe DS9 is little different darker than the others, but its too old. the characters are non-realistic as in all star trek series.
stargate... well, still in top 5, great effects, and characters, but it doesnt build up the mysteries, the secrets, and also it has also everytime a happy-end
farscape - for children from 16 to 19
babylon 5 - too old, but great