Since I’m a naval buff, for starters I’ll toss out Glenn Ford, who played Raymond Spruance in Midway. Pretty close IMHO.
Well, they weren’t playing real people, but Gert Frobe looked perfect as Auric Goldfinger, and Lotte Lenya was ideal as Rosa Klebb back in the first Bond movies.
And, despite what you might think of the movies, I think Robin Williams was the ideal choice for popeye, and Shelley Duvall as Olive Oyl. Matthew Lillard gets a mention as Shaggy in Scooby Doo.
I totally agree re: Popeye, CalMeachem, especially about Shelly Duvall. She was spot-on as Olive Oyl. My whole family loved that movie, BTW.
Most likely it’s becaiuse we just saw it this past week, but Josh Brolin nailed GWB, especially the walk and the speech patterns. There were moments when it was hard to see Brolin.
I’ll give it more thought, too. Robert Duvall was convincing both as Stalin and Eisenhower. Go figure. And Gregory Peck as MacArthur was uncanny as well.
Bernard Hill as Captain Smith on the Titanic.
I thought Denzel Washington was a decent physical match for Malcolm X.
I agree with Shelly Duvall and Matthew Lillard, too. A huge NON-match is Casey Kasem and Shaggy 
I nominate Stephen Fry as Oscar Wilde. They both have the same long face, pouty eyes and manner of speech. Bonus points for Fry being gay (and a huge Wilde fan to boot).
Don’t forget John Goodman as Fred Flintstone
Oh yeah I forgot Geoffrey Rush as Peter Sellers. Uncanny.
Or even better: Babe Ruth.
I can’t find pictures to back it up, but of the actors who have taken a crack at doing James Dean, Stephen McHattie30 years ago was damn close. More so than James Franco more recently although Franco was convincing, too.
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Bob Hoskins looks so much like Nikita Kruschev in Enemy At The Gates it’s scary, probably more so if you´re Russian.
Ed Harris as John Glenn
Scott Glenn as Alan Shepard
Sam Shepard (on the left) as Chuck Yeager
George “The Animal” Steele’s casting as Tor Johnson in Ed Wood was truly inspired.
George C. Scott and Patton. It wasn’t an exact match but it was pretty good.
I don’t know what it is about Ed Harris, but I thought he was a pretty good match for Gene Kranz, too.
Popeye was filled with good matches. In addition to those mentioned, Richard Libertini was perfectly cast as Geezle. No one commented because you needed to be familiar with the comic strip, not just the cartoon.
Dan Ayckroyd was perfectly cast as Vic in Neighbors. Many critics thought he and Belushi should have switched part, but Ayckroyd matched the book’s character description perfectly.
Mickey Rourke was perfect as Marv in Sin City