Unlike the others, TOS is not an ensemble show. It's the Kirk, Spock and McCoy show with prominent, memorable recurring character Scotty and a bunch of extras.
Sadly, these bunch of extras are the most insanely entitled in TV history and Takei in particular has made a career out of attacking Shatner and causing drama for profit.
That's not what the word "extras" means. Extras are background players who have no scripted dialogue. Extras are the people sitting at the side consoles and never saying anything, or the security guards standing by the bridge doors, or the yeoman who hands Kirk a clipboard to sign and doesn't say anything, or the crowd of citizens walking past in the background on an alien planet. Takei, Nichols, and Koenig were contracted semi-regular actors.
Which is unfair. Shatner was the main character, the star and the fact audiences loved his performance and were drawn in by it is the reason we're all here.
Actually Nimoy hugely eclipsed Shatner and the others in popularity, getting more fan mail than the rest of the cast combined. Roddenberry and Shatner both had to struggle to keep Kirk central to the show when the audience and the network were pushing for Spock to be the center of attention.
Since leaving Trek, Shatner has given award winning performances elsewhere, starred in a successful TV series and pioneered the undervalued music genre of spoken word.
He's hardly the only one. Nimoy starred on Mission: Impossible
for two seasons, hosted In Search Of...
for several years, and later went on to a successful directing career. Walter Koenig did extraordinary work on Babylon 5
. George Takei has done lots of character acting and voice work as well as performing on stage. And so on.
And you're not only underrating the others, you're underrating Shatner as well. He's had more
than one successful TV series since ST, including T.J. Hooker
and Boston Legal