I say DS9 but that could change with my mood.
DS9- I liked the way they fully enriched the Alpha/Beta quadrant, we got to see the Cardassians, Klingons, Romulans, Ferengis, Bajorans etc. as fully fleshed out civilisations. Political intrigue, space battles, cool ship (Defiant) etc. that is my thing. Garak and Quark were truly awesome non federation characters.
TNG- This had some great episodes but I prefer the darker/grittier tone of DS9. Apart from Picard the cast could be pretty bland. The Borg as a concept and Q as a character were highlights.
TOS- This is good only because it introduces the icons (Kirk, Spock and McCoy), I prefer the original cast in their movies much more. Probably way too far before my time.
Voyager- Totally wasted concept, the clean cut Federation crew and the traitorous terrorist Marqui members should have been at each other's throats the whole time, the tension and the hopelessness of their situation should have been off the charts. Chakotay should have been a total badass. Instead it was happy families. Disappointing.
Gay cowboy theme song- I never really gave it a chance, wasn't interested in a prequel series at all.
Animated series- Never watched it.