I think I would keep my opinions short and sweet.
Classic and campy, yet it never gets old for me. It's the original! You have to appreciate the original. Especially characters like Kirk, Spock, Bones, and Scotty.
More strict and classy than TOS, yet less believeable. A great show though. However, if it weren't for the odd-one-out character of Data I don't think it would be half as good.
More juicy storylines. More flawed, yet loveable, characters. Not as many cardboard-cut-out, Starfleet perfectionists. An overall feeling of freshness. Hardcore people trying to do the right thing. Some "magical" elements with the Prophets and the Shapeshifters. Many different species. Long story arcs. I have nothing bad to say about this show.
My personal favorite (close to DS9 though). Funny and takes itself less seriously than TNG. Character arcs are weaker than DS9, but stronger in my opinion than ENT or TNG with the exception of some TNG characters like Data. I love the humor, sarcasm, and wit of Seven and the Doctor. I have a feeling of respect for the entire crew the whole time I watch this show because they are alone without anyone to rescue or help them. Being strong in the face of adversity and accepting what life has dealt you, yet never losing hope. I enjoyed being introduced to new species and, despite what some may think, I enjoyed the time travel episodes and the Borg arc.
Started out shaky, but ended up being a very decent prequel to the entire "prime timeline". I enjoyed the throwbacks to more of today's type of technology. The showing of actual space-travel and human development events during the opening sequence is appealing. Raw, simpler, and discovering new things in an unexplored galaxy. Yet there was not enough character developement. Also WTF was up with Trip dying. Not cool. Not cool at all. :/