TOS: The weakest show in the series, cheesy, repetitive, at time bizarre, frequently average, often poor, occasionally brilliant...yet...fun, engaging, entertaining with an underlying brilliance. The TOS movies were the pinnacle of this show.
TNG: What TOS wishes it was. TNG was intelligent, philosophical, touching, funny and clever. A beautiful look at the human condition, what we are, what we should not be and what we can be. One of the greatest TV shows ever made.
DS9: Dark, gritty, yet with a streak of realism that all the other shows lack. It was far less subtle than TNG, it was more controversial, it injected realism and humanity into an idealistic world and showed how the Roddenberryesque vision would work for real. The pinnacle of the series. So so good.
VOY: Crimnially underrated. Similar to TNG except perhaps less....rigid. Voyager also had a DS9 like ability to be controversial. VOY was a family, it was dramatic, it was touching and so very very watchable. Funny and serious, adventurous and wonderous. On its day, it has the potential to be the best.
ENT: Functional. From the ship to the jumpsuits, ENT had a feel
I simply adored. There was something 'realistic' about the whole show. It was capable of being gritty, dark and yet funny and light hearted. ENT is a weird mix of all the previous series.