I'm a fan of
Robert, the only reason I wouldn't use it myself is that I like
Robin more.
Robert was my grandfather's name and he went by
Bob. But I think
Rob and
Robbie would come just as naturally. It depends what the user and their acquaintances. I prefer
Robbie.
I find -bert endings handsome. They remind me of Schubert, which I think is a beautiful surname.
Hubert is my fave, probably because it's closest to Schubert.