IYO = In your opinion.
For me its
Christian Bale - Batman
Hugh Jackman - Wolverine
Robert Downy Junior - Iron man/Tony Stark
Chris Hemsworth - Thor
The professors in both Jurassic park and Back to the future.
Patrick Steward - Prof X
Sir Ian Macellen - Gandalf
To me those guys cannot be replaced without the feel of the movies being destroyed.
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Steve Coogan - Alan Partridge