I think DS9 had great characters, more importantly though, it had great character dynamics. Its like the great relationship of Bones-Kirk-Spock, but for the whole cast, plus recurring characters. There was so much variation in the interactions, and I think thats where DS9 is the superior trek. Other characters, particularly Picard, were just as interesting, but there was never much going on between the characters in other series, so they felt kind of isolated. On DS9, every character relates to every other character in such a fun and unique way. Everyone seems so realistic, and it gave the stories a lot of depth and substance.
I dont think its fair to say that DS9 is the best though. Its dialogue was never as good as TNG or TOS, and while it had some great shows, it still experienced some crushing lows. The whole Dominion War/Prophets thing ended so poorly that I find it sort of makes preceding episodes worse. Im also disappointed that it became the type of show where every
female character needed to become someones girlfriend. Ezri and Bashir? Garak and Ziyal? Odo and Kira???
I sort of just view TOS/TNG/DS9 as equally good at what they were trying to do; its difficult to compare things which are so fundamentally different. VOY and ENT started backtracking, instead of seeking their own styles, and screwed up on all possible levels.