Never saw Merman or Martin. Saw Channing several times in "Hello, Dolly!"
I would have liked to have seen Merman in "Something for the Boys" or "Anything Goes" -- or anything of the Cole Porter shows. And Martin in "Leave It to Me!" doing "My Heart Belongs to Daddy" would have been great.
As for "Gentlemen Prefer Blondes," I saw a bus and truck tour years ago with Morgan Fairchild, who was Marilyn (or at least tired to be), and Debbie Gravitte, who was great.