Captain James T. Kirk, the role that William Shatner is best known for, would very likely be on the "Nerd Mount Rushmore." (Who else makes it on the "NMR"? That's another post.) In watching Classic Trek, there are three different William Shatner's:
1) The overactor. This is the William Shatner that everyone makes fun of. The ham. He's the one that places odd pregnant pauses into his speech patterns, emphasizes words in slightly odd ways, and makes exaggerated facial expressions and gestures. Sometimes this William Shatner is jarring, but he has quite a bit of flair that can't help but be admired. If nothing else, he brings energy to the character and infuses it with quite a bit of fun. This helps to sell the "funny" episodes of Classic Trek like "The Trouble with Tribbles" and "A Piece of the Action."
2) The operator. I call this Shatner "the operator" because all he does is phone his performance in. There are some bad episodes of Classic Trek and they are not made any better by a Shatner who knows it's a stinker. And Shatner also really works well with DeForest Kelley and Leonard Nimoy because they tend to balance him out. So when he's forced to work with characters he doesn't have good chemistry with, say Koenig and Nichols in "The Gamesters of Triskelion," he just sort of takes control and doesn't have anyone to reign him in.
3) The officer. Sadly, this is a Shatner we probably didn't see enough of. This Shatner was pretty exclusive to the early first season. He was a serious career captain who obviously cared about his crew. But he was actually pretty strict and would cut anyone of his crew down to size if they didn't perform to a level he expected. He loved the Enterprise and would do anything to protect her. He was human and made mistakes, but he owned up to them and made things right. Still, he had a reserved, calculating side that came out in "The Balance of Terror" or "The Corbomite Manuever" that showed why despite his faults, he was in charge. It's a shame that this Shatner didn't show up for more episodes.
When it came to the movies, Shatner showed his acting chops the best in Star Trek II. On the director's commentary for II and VI, Nicholas Meyer reveals that the way he got what he wanted out of Shatner was by doing scenes over and over again so that all of "the overactor" stuff got out of the way. And the scripts were strong enough that "the operator" doesn't make an appearance in either of those two movies.
I think Shatner is some sort of genius and doesn't get the credit that he deserves for the success of Star Trek. Imagine a Star Trek as popular as it is today if Jeffrey Hunter had stayed in the captain's chair.