Well it's in my sig but for funsies...
1. DS9
My favourite TV show of all time. I adore it. I've already written a lot on why I love it (see sig) so much. The main cast, the guest stars, the ship, the station and over-arching plot and the way it manages to be darker but still faithful to Trek's positive core is just wonderful.
2. TNG
Brilliant. So watchable. Fun. Positive. Trek done right.
3. VOY
Criminally underrated show. Mulgrew, Dawson, Picardo & Ryan are all excellent in it. On its day I may actually love Voyager most but its inconsistency lowers it.
4. ENT
Poorly thought out, poorly executed. 2 seasons of...nothing. Then, 2 seasons that were brilliant (and most fans had tuned out and the network was sharpening the axe). ENT, once it found its feet, was a great show. I loved the ship, the atmosphere, the uniforms, the tone...sadly it took too long to get going. I'm still salty we never got a 5th season to properly wrap it up and say goodbye.
5. TOS
Great show. Adventure, fun and the start of something iconic. Sadly, in the 3 seasons, way too many episodes are duds and there only seems to be a handful of actual plots repeated over and over (super powered being, shuttle crashes etc). I think TOS truly came into its own in the movies where it destroyed TNG. In every other way though, TOS is just inferior. I still love it.
6. DISC
I still haven't finished the first season. I am unconvinced. It may be the first Trek I don't love.