Warren Clarke as Det.Supt. Andy Dalziel in the TV series Dalziel and Pascoe. I read the books before seeing the series and he was absolutely the image I had for the character.
Seconded. Lucian Msamati as JLB Matekoni and Anika Noni Rose as Mma Makutsi are equally spectacularly cast.
Interesting note from Wikipedia on the casting:
“Character’s build” in this case meaning “fat”.
Since TV shows have been brought up, Jim Parsons as Dr. Sheldon Cooper on “The Big Bang Theory.” That is not an easy role to play, and he does it to perfection.
When they were putting “3rd Rock from the Sun” together, someone asked “Who would be good as Dick Solomon.” Someone said “John Lithgow” and that was it. Nobody else was even considered.
I’d toss in Charlie Sheen as Charlie Harper, but he’s only playing himself.
I couldn’t imagine anyone besides John Malkovich playing John Malkovich in Being John Malkovich.
Oh, I don’t know. I’ll bet Meryl Streep could do it. 
There are some films that are perfectly cast, top to bottom. I’d put “Cold Comfort Farm,” and “A Streetcar Named Desire” in that category. I’d almost say the same of “Casablanca,” but a wet dish-rag could have filled in for Paul Henreid.
Someone other than Paul Newman might have been able to play Fast Eddie Felson in “The Hustler,” but only Jackie Gleason could be Minnesota Fats.
OTOH, only Paul Newman could possibly be Cool Hand Luke.
Oh yes! I read the book before I saw the movie and he was exactly what I had pictured.
Voice only, but:
Paul Lynde as Templeton the Rat in Charlotte’s Web.
Steve Buscemi as Templeton the Rat in Charlotte’s Web.
Yeah, I was initially thinking they might get Nathan Lane, but Buscemi was totally perfect for that!
Two different portrayals of the same character- the first is near iconic but the latter is faithful to the book-
Dwight Frye as Renfield in Lugosi’s DRACULA
Tom Waits as Renfield in Copolla’s DRACULA
and heck, capturing the Frye essence in a book-faithful portrayal, a third is Jack Shepherd as Renfield in the BBC Jourdan DRACULA.
I personally thought that Audie Murphy really nailed the character of Audie Murphy in To Hell and Back. He really seemed to grasp the character. He may have been a bit old, but not that bad for Hollywood.
Johnny Depp as Captain Jack Sparrow.
Peter O’Toole as Henry II both times.
Peter O’Toole as Lawrence of Arabia.
Peter O’Toole in My Favorite Year.
Russell Crowe as Maximus.
Most of these were already mentioned, but what the hell:
Patrick Stewart as Proffesor X - Perfect look, perfect voice. Oddly enough, he’s a case where our pre-existing familiarity with him as Captain Picard just made it easier to see him in a similar role.
JK Simmons as J Jonah Jameson - Again, perfect look and perfect voice. After being so familiar with his role as Vern Schillinger in Oz, seeing him play such a completely different character in such an over-the-top comic character is just plain awesome.
Kevin Conroy as Bruce Wayne / Batman - His voice is iconic for the role and he’s able to achieve very distinct voices for that interpretation of Batman where so many others either don’t even bother (George Clooney, Val Kilmer, Adam West) and the few that try do it poorly (Christian Bale, Michael Keaton)
Brad Pitt as Tyler Durden - Sure, not the greatest actor, but I just can’t imagine anyone else in that role. Even more, the sheer irony of his sex symbol and pop culture stature in Hollywood when contrasted with the principles of his character in the movie just add that extra bit of believability to the character because (spoilered just incase anyone still hasn’t see the movie)
were I the Narrator, I couldn’t imagine many other people that I might have subconsciously modeled my “ideal” alter-ego after.
Most of the cast of The Princess Bride. Particularly Cary Elwes, Mandy Patinkin, Wallace Shawn, Andre the Giant, Billy Crystal, Peter Falk, Christopher Guest, and Carol Kane. Without the exceptional cast, that movie would be pretty forgettable.
What Ever Happened to Baby Jane?"
I cannot imagine this film being made with anyone but Bette Davis and Joan Crawford. And Victor Buono was a perfect fit too.
Timothy Spall as Scabbers the Rat (sort of) in the Harry Potter movies.
Also, Michael Clarke Duncan as Wilson Fisk (AKA The Kingpin) in Daredevil. I know some people carped about the fact that Duncan is black and the comic Kingpin is white, but honestly, who really gives a shit? In every other way, Duncan IS The Kingpin.
Great one!
I alway thought that actor/character inspired Stephen Root’s casting/part in “NewsRadio”, which I’ve only actually seen once, (aside frome bits here and there), and it was the episode after Hartman died. So I’m not sure that’s accurate.
Also in The Green Mile, Doug Hutchinson as Percy Wetmore.
John Malkovich in Being John Malkovich
Max Schrek in Nosferatu