Cav is superb, certainly. It is always dangerous to compare athletes from different eras because tactics change, training methods change and so on. Back in the day, there were fewer dominant teams that delivered their man to the line with an organised lead-out. I think Rik van Looy more or less invented that with his "Red Guard" in the early 1960s. He was a hell of a sprinter, though.
I've been following pro racing since the 1970s. If I had to pick one who was clearly in Cavendish's class I'd go for Freddie Maertens. Twice world champion, winner of eight stages of the TdF in a single year, very very fast.