If we are talking qualification, Senna and Clark are good shouts. Fangio should probably be mentioned as well. Today, Vettel has better stats but I suspect Hamilton is marginally quicker over 1 lap. From the Schumacher era, I'm not sure if he was faster than Häkkinen. If we are just talking pure speed whether it be in qualification or the race, one could throw in Gilles Villeneuve as a wildcard.
Quali is an interesting one. Some drivers find that bit extra when they're really comfortable with the car whereas others are more pliable and can get the best out of what they've got.
I think it's fair to say if they both had a car set up just as they like it Hakkinen was in fact generally quicker than Schumacher. Mika liked a 'pointy' car - one that snapped into corners and when he had one he was lightning quick. Schumacher by contrast could wrestle with a car and force a fast lap out of it and he could hustle for the whole race if needed - poor tires, back-markers, poor balance etc - so was way more consistent generally. Senna, to me, was even better than Schumacher at this but Schumacher had a single-minded determination which probably made him like a more disciplined although slightly less talented version of Senna. The Prof... what can you say, the man was the benchmark for great driving. Whether that was the quickest way around a track for a whole race will be up for debate forever.
(also, when Hakkinen left McLaren a reason they brought Raikkonen is he suited the inherent tendencies of McLaren's car design leanings so their historic set-up data would be more applicable for him off the bat.)
Comparing the current F1 drivers to guy like Prost/Senna and further back to Fangio/Clark etc is a little like comparing tennis players to previous eras. There are details which make the comparisons difficult even if the general talent can still be debated. I do think that Vettel is the Schumacher of this era - he's a good driver and very disciplined which is what is needed but he has had the best car for ages. All things being equal I wouldn't consider him a better driver than Alonso, Hamilton or Raikkonen and maybe even a few others.
Vettle demonstrates perfectly how the car makes so much difference in F1 - probably moreso than ever. There's no way the best driver would be winning in the 6th best car except in rare anomaly events (crashes, rain-affected races etc).