Robert ‘Guiscard’, because very little beats “Robert the Weasel” as a medieval nickname ;).
I just learned that the Scottish king in the Bruce and the Spider poem was a Robert.
Robert “Rob” Roy. I’ll drink to that!
Robert H. Jackson
Robert Guillaume. Loved watching Benson as a kid.
Robert Mitchum
Robert Desnos
Robert Nesta Marley
You win the thread.
Hey, uncle Bob! Long time no see! 
Rankin
What about listing some scientists?
Robert Oppenheimer (okay it’s J Robert Oppenheimer)
Robert Hooke (discovered the cell)
Robert Brown (discovered Brownian motion)
Robert Boyle (Boyle’s Law)
Robert Goddard (rocketry & propulsion)
Robert Bunsen (Bunsen burner & many other discoveries)
Rules of Order
Robert Benchley
Robert Preston (with a capital P)
Robert Hall
Robert J. Flaherty
Robert Browning
Robert Burns
Robert Louis Stevenson
Robert Altman
Robert Donat
Robert Walker
Robert Armstrong
Robert Enrico
Robert Frost
Robert Herrick
J. Robert Oppenheimer
Theodore Robert Bundy
(OK, the last two aren’t favorites, so much as people whom I find very interesting.)