DS9 is probably my favorite overall, and a big reason for that is the characters. They all progressed incredibly well and the cast was fantastic and balanced.
I also feel DS9 was the last "great" ST series, and was the only good spinoff (I don't really look at TNG as a proper spinoff since it was a resuming.) Having gone on to a current watchthrough of Voyager, I think it had potential to have similar character progressions, but the writing totally wasn't there. I really wish that it and Enterprise would have learned something from DS9.