ANSWER
Carl Reiner, Johnny Carson.
Series creator Carl Reiner starred in the pilot episode, but CBS executives wanted someone else in the lead role. Legend has it they thought he was "too Jewish", but whatever the reason they set about recasting the role of Rob Petrie. Johnny Carson was first considered for the role, but ultimately it went to Dick Van Dyke, and Johnny went on to host The Tonight Show the following year. The rest is history.
WHO GOT IT RIGHT: JP Weigt, Ryan Scannell, Carolyn Moody, Brian Bement, Christine Severen, Kathi Reynolds, Andie J, Mark Collins, Peter Phelan, Marc Quinlivan, Robin Campbell, and Bob Milligan.
