I think it was Adam West that said he copied his … interrupted… style of speaking… from Shatner… because… it gets you… more screen time.
If you can find the pilot for Alexander the Great that they both were in, by all means devote an hour (or maybe more; I don’t remember) of your life to watching it. It’s a hoot and a holler! 
I once did Eastwood (and a bunch of other voices) in a series of radio commercials for a Dutch carpet company. Unfortunately, they were never aired because the company rep the station was dealing with was a humorless dick:
**FADE IN
[FAST JAZZ PLAYS IN BACKGROUND]**
POSH BRITISH ANNOUNCER: And now, B _ _ _ _ _ and S _ _ _ _ _ _ _ present Dirty Harry for M _ _ _ _ _ _ _ Carpets.
HARRY: A man’s gotta know his own limitations … when it comes to buyin’ carpets. Now, you’re probably wonderin’: Should I buy M _ _ _ _ _ _ _, or some other brand? Well, before you answer, you wanna ask yourself just one question: Do I feel lucky today? Well, punk, do ya? Huh? Do you feel *lucky *today?
ANNOUNCER: If your carpets are as important to you as they are to [A CLIENT], you should buy M _ _ _ _ _ _ _.
BLACK PERP: Hey, man! Ah gots t’know!
[PAUSE]
BLAM! BLAM! BLAM! BLAM!
Besides Eastwood, I did these commercials as Queen Elizabeth, Mr T, James Bond (Sean Connery, of course), Elvis Presley, and Captain Kirk. I wanted to do one as Beavis and Butt-Head (“Hey, Beavis! Have you ever … ‘done it’ … on a carpet?” “Huh-huh! Carpets are cool!”) and one as Homer Simpson (“If it’s good enough for Donald Trump, it’s good enough for me!”), but we somehow never got around to it. 
Something similar happened when I did Bullwinkle and Dudley Do-Right in a series of commercials for a Canadian-themed bar; again, they were never aired.* But I swear, when I listened to the playback I never would have guessed it was me if I hadn’t known!
*The owners wanted a parody of Wild Kingdom, which I was also happy to do, but the script they handed me ran more than a minute and twenty seconds. I tried to tell them beforehand that there’s no way this is gonna be around thirty seconds, but would they listen? NOOOOOOOOOOO!!!
Bob Hope was tough to impersonate and Thomas was the first to really get him down pat. Rich Little tried but always seemed to have trouble with it. Sinatra’s speaking voice also seemed difficult for impressionists before Joe Piscipo and Phil Hartman.
On a related note, there are a lot of well-known people who are (or were) seemingly impressionist-proof. For example, Paul Newman was a recognizable star for over 50 years but I have never heard anyone even try to impersonate him. The same is true for Warren Beatty and Robert Redford.
That’s three guys known more for their faces than anything else:) They really don’t have distinctive speech styles, either in the sound of their speech or the words they use. Newman is more dynamic overall, the other two are like models.
Pollack’s Shatner is amhoot, but mostly because of the exaggerated gestures and facial expressions.
I do think Shatner gets a bit of a bad rap as an actor, though. The guy was trained as a Shakespearean actor in a theater company, where you have to learn to project and enunciate very clearly to he heard and understood. Sometimes that came across a bit too much on screen. But Shatner is/was capable of turning in a great performance when he tones it down. His Kirk was light years better than Chris Pine’s. Shatner’s Kirk exuded intelligence and command presence in ways that Pine has never managed, even though Shatner was close to the same age as Pine when playing Kirk in TOS.
William Shatner is and always will be the one and only Captain Kirk, now and forever, reboots be damned! Period, end of sentence!
His idol and model-to-emulate was Laurence Olivier. If you watch Olivier in, say, his version of Henry V, it’s easy to see the influence he had on Shatner.
It’s also interesting that Olivier himself impersonated a number of people in some of his performances (albeit non-famous ones). His portrayal of Richard III was vocally based on a strict and pedantic teacher he was unfortunate to have in school.
Getting back on topic, Lenny Bruce, from what little bit I’ve heard of him, was a surprisingly good impressionist. He also liked to do obscure or less-obvious celebrities (e.g., character acter George Macready sniffing glue) just to throw people off.
Peter Sellers was also an excellent impressionist. He was impersonating Michael Caine long before anybody else was. In addition, he could do a dead-on Pavarotti even though he was unable to sing a note in real life.
Jamie Foxx is also a terrific impressionist. Here he is with Jimmy Fallon doing musical impersonations. The last one just about had me on the floor.
Yeah, I stand corrected. How could I forget about Rich Little? But, to me, Robin Williams is funnier. He didn’t just do impressions; he brought them to life!
Best Shatner I ever heard was of all people the “Will and Grace guy” in Free Enterprise. I do a very good one as well. To my ear Eric McCormack did Shatner as he sounds not as others do an impression of him.
Though hearing others do an impression is helpful, and finally…I don’t know if others do this…i usually need a key phrase to get into an impression. “Risk is our business” is required if I’m trying to nail Shatner as he really sounds.