The Sisko ;)
TNG, Voy & Ent would had been tougher to choose. But with DS9; while the other cast members are great and important; Siskos Char is the first coming to mind.
I'd actually say it would be way easier with Archer in Enterprise. Enterprise, much like TOS, had the three main characters and the rest as support, but Archer felt more like a main character even among those three than Kirk did in TOS.
But in DS9? I get where you're coming from, Sisko had the whole "Emissary" hting going on, but every character in that had just as much screen time and focus given to them. In that regard I think DS9 was the most "evenly spread out" series of them all- Voy was like that untill season 4, and since then Seven of Nine, Janeway and the Doctor definately got most of the screen time. TNG was pretty even, but there was still more focus on Picard, Data, Riker and Worf than geordi, Crusher or Troi.
Still none of the shows really had just one main character. Now, I'm not saying that such a change in the shows dynamic for DIscovery is a bad thing, we'll see about that. But if feels a bit weird.