Sean Connery, The Untouchables.
Great one. That role seems to have been made for him. There's another one that seemed to fit him like skin, Family Business. Also starring Dustin Hoffman and Matthew Broderick. Broderick was good, but Hoffman, arguably the best actor of the three, stood out as being not good in that part.
I'm far from an expert on acting. I've never done any, and to me, it strikes me as one of those things that's only made to look easy after a lot of training, but that's very easily done badly.
Sometimes I do feel like an actor, in that there's some transforming from character to character when writing fiction.
This thread was titled ironically. Meaning, I don't think any actor has any one best role and therefore it's all a matter of opinion. Paul Newman, for example, was great in so many things, (The Verdict, Cool Hand Luke, Harper) as was Jack Lemmon, (Wine and Roses, Odd Couple, Glengarry Glen Ross) that maybe the best thing that could come out of this thread is that people learn about movies and performances that they were unaware of.
The Untouchables also starred Andy Garcia. I wonder how many people saw him in a fairly obscure movie called The Man From Elysian Fields, where he plays, are you ready? A published but struggling novelist who becomes an escort to support his family. The escort service is headed by of all people, Mick Jagger, who's quite good in it actually.