My list:
5. ENT - Interesting idea for a prequel series, but ultimately it became it's own little setting and didn't feel very "Star Trekish". Most of the characters were uninteresting.
4. Voy - Not that this was bad series - it was a great one - but it felt underwhelming. The plot had a great start and became involving at first, but then was very quickly forgotten and turned into an excuse to just have another ship exploring new worlds. Some of the characters were great.'
3. ToS - One of the most memorable television shows of all time - everything was superb.
2. DS9 - The most unusual of the Star Trek shows; rather than an episodic adventure about planet-eating snowflakes and evil space robots each time, the series was a saga of two coalitions of races from different parts of the galaxy uniting, helping, betraying and backstabbing each other in what was undoubtedly the darkest series of them all. The "GoT" of the 1990s. Believable characters, excellent writing, all contributed to make this one of my most favorite shows.
1. TNG - The true sucessor to ToS, with the best characters and all the goofier sets and crazier ideas. It. was. LEGENDARY!
Okay, I'm done reminiscing now. *Sniffs with nostalgia*