Alright....I've recently acquired all 7 seasons of TNG and DS9 and I'm working on Voyager. In a year or two I'll come back and let you know which I think is best after I've officially seen every episode of each
I've already seen maybe 85% of TOS and TNG. The only way I'll change my mind about TNG being best is if Voyager or DS9 REALLY impress me. I doubt they will though - the acting and production value of TNG is top notch. It's the only Trek series that got me to actually CARE about the characters - a perfect blend of Sci-Fi (the part TOS did great) and drama.