Never really considered it before, but I think Guy Williams would have made a pretty awesome Enterprise captain.
I always thought he was a really compelling and charming presence on screen, and thought it was a shame we saw less and less of him as the show went on.
I think Shatner was a little more accomplished as an actor, but Williams would have been very good in his own way. Both men made a conscious effort to move with a cool, catlike, athletic gracefulness. Maybe Williams would have played Kirk overall in way similar to Shatner. Leonard Nimoy might have found the same kind of "space" in which to fulfill Spock's potential.
On the other hand, it's doubtful that the Spock role would have ended up so similar if Mark Goddard had gotten it.
Photoshop by Charles Thaxton.