My rankings each series (my opinions only, no need to take offence ).
1- TNG ... Were I started. Love the warm lounge room feeling I get.
2- DS9 ... a nice gritty change from differnt perspective of life in Starfleet.
3- Voy ... A fresh clean start after DS9. Just get rid of the Captain :p
4- Ent ... Cool concept to start with, but the Xindi storyline was a total train wreck.
5- TOS ... Love the movies, but series makes me cringe - total rubbish.
DS9 got much better with time and it ended up being perhaps the best Star Trek series, in my opinion. Starting around the time of the Klingon-Federation war, the show just got better and better, in my opinion. Mostly very talented actrors/actresses, good story lines, excellent special effects, a lot of suspense, but still "Trek." Was it perfect, no. But it was pretty darn good.
I remember a very sad episode that I though was fantastic. Its the one where Sisko keeps disappearing in front of his son. It goes through Jake's entire life with his father reappearing here and there, and shows how devastated Jake was. It was a very emotional episode