Star Trek is one of these weird franchises for me in that I kind of like (almost) all of them, but I don't really love any of them at the same time. Here is my overall ranking based on what I've seen:
1) Voyager - I actually watched the entire first season, and I liked the entire cast. I liked the concept of them being lost. After the first season they changed the time slot on me and I stopped watching.
2) Enterprise - I watched a lot of the first season. My friends would tape it, and we'd watch it together. It came on really late on a weird night. At a certain point I stopped watching it with my friends, and I missed the rest of the series. I really like Scott Bakula and the rest of the cast is decent too.
3) Original - I like the original cast the best out of any generation. If we were including the movies, then I might rate this cast as the top. However, just going by the series it rates lower. The cast is still great, and Kirk/Spock/Bones is like the holy trinity of Star Trek. But something about the plots or execution makes these shows boring. Not sure why. Also I've seen the tribbles episode like 4-5 times while there are lots of episodes I've never seen once. Not sure how or why that happened.
4) Next Generation - The only characters I like are Picard and Data. Worf is ok. The rest can be jettisoned into space. The plots are better in this series, but I really don't like the cast. I mostly watched this, because I had a friend that really liked it and it happened to be on when were hanging out together.
5) Deep Space 9 - I could never get into this show at all. To me Star Trek is about exploration. This is just some people hanging out at a space station together.
Those are all of the series I've seen. I probably got attached to the original cast more from the movies though. Star Trek 4 is a great movie that I've seen several times and I'd still enjoy watching again. Star Trek 1 is also good even though the pacing is slow. The other movies with the original cast are decent. I really like the original cast, but often the writing/directing/editing doesn't do them justice.