In order of preference:
1. Deep space nine. Great and deep story arcs & insight in characters from alien worlds, dark atmosphere. Sometimes somewhat too much soap-like though.
2. TOS - I used to have TNG second place but these have switched places lately. TOS characters are just slightly more vibrant to me (of course Kirk/Spock / McCoy, but also scoundrels you can't really hate, like Mudd).
3. TNG - Great stories, but sometimes too politically correct, and too much pluche (so to speak). Everybody loves classical music onboard the enterprise-D ?
For me, the first three are close together, the largest gap in preference is between 3 and 4.
4. ENT - going before where TOS and TNG went. Or something like that. Still liked the concept of the 22nd century alpha quadrant, though. A lot more could have been done with it, I think.
5. VOY. Not bad. Just average. Still a lot of standout eps I like to watch.