My vote would go to Freddy "La Dynamite" Maertens. In five Grand Tour starts he won 4 points competitions (failed to finish the 1977 Giro d'Italia) and won 35 individual stages (Cavendish is 30 stages from 10 GT starts with 2 points competitions). Maertens' other wins include two World Championships, Gent Wevelgem, Amstel Gold, Championship of Zurich, GP Frankfurt, Paris Tours and Het Volk. No monument wins, but top 5 finishes in all of them. He was more than just a pure sprinter; he won the prologue and two time trials in the 1976 Tour plus GP des Nations and won the 1977 Vuelta overall as "training" for the Giro d'Italia. He was in 2nd place overall in that Giro when he crashed out and did have aspirations to win the GC there. He also had big hopes for the Tour de France in 1976. He and Michel Pollentier had spent the previous winter training on Alpe d'Huez to prepare for the stage that finished there (who'd have thought; reconnaissance training back in the 1970s?).
I digress a bit with the all-rounder stuff. In the '76 Tour, Maertens was virtually unbeatable in a bunch sprint (I think Gerben Karstens got the better of him twice) which made him the equivalent of Cavendish in terms of sprinting dominance. In time, I think Cavendish will eclipse Maertens' record but for now, IMO Maertens is the best sprinter ever.