Tough call, because I like each one for completely different reasons
The original ST because of the Kirk, Spock, and McCoy dynamic absolutely made that show.
I like ST: TNG because of Data, the Borg (as stupid as they were used), Q, the Galaxy class Enterprise, and several episodes are among my top favs.
DS9 - totally hated that show. But I did like Cap. Sisko, Garak, the Defiant, Dr. Bashir, Section 31, and the Jem'Hadar.
Voyager.... a lot of inconsistencies with this series, but I liked Janeway, Tuvok, the EMH, and 7 of 9. The ship wasn't all that bad either.
ST: Enterprise.... I only started to like it towards the end when they really got better at taking chances with their writing, and the characters. My fav episodes were the Augments, the Klingon Augments, and Tripp's clone.