It's the trusty old 747!
It was tested in a dive to M 0.991 in the original certification tries, just to test for mach tuck.
FWIW, I've seen M 0.88 fairly sustained in the cruise .... we were in a hurry!
The fastest cruiser though, I think, was the Convair 990 (??) Did something like M 0.9 sustained.