Sergio muthaf'in Toppi!!!! (gangstershit put me onto this guy..he's worth it!)
Google it. you need to work a bit for this amount of awesome, I ain't giving it to ya! :D
Also top of my head from a range:
Winslow Homer
John Berkey
Tibor Nagy
Theo Prins
Marko Djurdjevic
Jaime Jones
Alex Garner
Craig Mullins
Kekai Kotaki
Wes Burt
Nox
Jason Chan
Brom
Masters:
Rockwell
Sargent
Frazetta
Bacon
All in no real order, although I do have a soft spot for Djurdjevic as the first time I saw his work it was a total watershed moment in my art career and started me on the path to becoming a digital artist myself. His work basically changed my life and enlightened me to a whole new career.