Well, many would view Spock as the main character in the classic series. And, aside from Picard, Worf and Data were the most popular in TNG. The Doctor and Seven of Nine (a human turned Borg drone) were probably the most popular in VOY, while Dax, Quark, Odo and Kira had their fans in DS9.
I'll grant you that Spock was part of the main triumverate of characters in TOS - but really, I think it would be hard to argue against the main-est character being Kirk. The actions of Spock and McCoy both really seemed to mostly revolve around him, up to and including playing "devil and angel on your shoulder" with him sometimes. Also, it is hard to lump TOS in with the rest of what I'm talking about, because getting a feel for ratings was harder back then, and even the ratings they had seemed to indicate that Trek was a bit of a niche market then - which is why it only lasted two years before getting cancelled (and then being allowed an encore season by demand of geeks like us.
)
Some of the things I'm about to say would be offensive to the characters that they are about, so sorry for that, but: Worf was portrayed, at heart, as a human (albeit maybe a samurai) looking in on "his" people and not truly understanding most of them. Data's character was almost entirely about trying to learn what it is to be human - which is still the human adventure, really. Ditto for The Doctor and 7 of 9, but also, as badly as Voyager was doing at the point where they brought Seven onto the show, I really kinda think that any "strategy" for ratings that involved them took the form of flailing about at a crap shoot. Dax was a long-lived human (or several) with spots - and hot, which always helps. Kira was a human with a funny nose from a civilization that seemed to me to be meant to be Israel with funny names. Quark was another Worf-like character, looking back in on his own race with human eyes - only instead of a samurai, he was a (probably offensive stereotype of a) Jewish merchant.
Odo has probably been the most truly
alien alien that Trek has seen as a regular castmember - and even then, his story still takes the form of human (okay, "Bajoran" - still human) looking at his own people.
And none of these has been the
main character - with the possible exception of 7 and The Doctor, because as I said before, Voyager kinda lost its way - pun painfully intended.
Angel was a human - vampires are still humans, just undead and cursed, but the main theme of his story most of the time was trying to get back to what it means to be human. Clark is an adopted human - he has powers and an alien background, but ultimately, he's a farmboy from Kansas. The rest of the characters that you mentioned don't really count because they were either single movies or movie series with episodes space a few years apart or more. Some of the popularity of those movies can be chalked up to theater-worthy effects, the general public's willingness to
occasionally indulge in something that they regard as silly or nonsensical, or a really good single script. But this isn't about what can make a good, single story. It is about what will make a compelling
series of stories that will keep an audience tuned in to a serial television program for half of each year (26 episodes) for 7 years. And for that, you need "the human adventure", it really does seem to me.
