In no particular order, and simply for thier content and achievements as tv shows.
1. BSG (Reimagined) - fantastic presentation and content.
2. Lost - great at keeping you hooked
3. Stargate SG1. - 10 seasons is a major achievement for a sci fi show.
4. South Park - Great comedy take on modern issues, never afraid to make fun of something.
5. TNG/DS9 - can't decide between which, they both had a great impact on the genre, and paved the way for future shows.