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.
The one which you have watched first / the most / at the right age. For me that's Babylon 5, not Star Trek. I still haven't even watched all the ST movies, let alone all the TV episodes. But I like the presence of Data, Spock, and Picard, so whatever series those characters are in.
Deep Space Nine, without Siskos irrational temper tantrums and dramatically terse voice.
Sisko (without the held back angry voice) was the best war Capitan. I still hold that I'd have been a better Capitan though; I'd have employed the people in a much better manner.
Garak would have been my full time advisor
Bashir would have been my science and technology advisor
Gul Ducat would have been my military advisor (bribing him into defection)
With these two in their correct spaces, I'd have been able to annihilate the Dominion and have assisted the Cardassians in their military expansion throughout the Gamma Quadrant, after having negotiated their pacifism in all but trade enterprises in the Alpha Quadrant and free economic passage (tariffs excluded) throughout for the UFP, Romulans, and the Klingon Empire
It had the best cast, with fleshed out civilisations and political intrigue.
The others were just ongoing adventures and Janeways happy families.