The reason the likes of Hamilton and Vettel have such vast numbers (and to an extent Schumacher) is because the teams are increasingly efficient. Reliability isn't much of a factor these days, and the likes of Mercedes and Ferrari are often a distance ahead of the opposition, especially during the hybrid era. Indeed since the start of 2014 Mercedes and Ferrari have won 89% of races, with the remaining 11% being won by Red Bull.
It's difficult to compare drivers across eras, but we can judge traits; things like control, aggression and consistency as well as speed among their peers.
Few drivers have had control, aggression and consistency as a trait throughout F1's history. I'd argue Senna lacked consistency for much of his career but so has Hamilton at times (especially when Button joined). Vettel lacks wheel-to-wheel skills, but in my mind he's undoubtedly one of the fastest drivers in the last decade, which he largely matched with consistency at Red Bull. Prost lacked aggression, but you could argue it was the lack of that trait that was key to him winning so much. Probably the closest to having all those characteristics is Schumacher.
Conversely there are traits that can't be compared. In the 50s, 60s and early 70s bravery could be found in all drivers, something that's largely missing in F1 now, where crashes often result in nothing more than a discussion of who was at fault. The reactions to crashes 40+ years ago were along the lines of "are they injured or worse?". In that regard none of the drivers in the modern era can be considered against the likes of Fangio, Ascari, Clark or Stewart.
Still in general Hamilton is certainly up there in the top 10 (and probably in the top 5) of F1's greatest drivers for me. Despite only having two titles I've no doubt Alonso is up there too. Vettel is divisive, but for me his championship stint at Red Bull is enough - I've said before several times that he could visibly do do things with that car that others simply couldn't - as Webber himself once admitted. He's not as rounded (or frankly as good) as Hamilton or Alonso now, but I think he's worthy of being on the list.
Equally there are question marks over the likes of Schumacher too, though how a seven times champion can ever be considered not great I don't know. Obviously he didn't always have the strongest of teammates but some of his on-track performances were stunning. His drive at Hungary to beat the McLarens in 1998 I regard as the best drive I have witnessed and his win at Monaco in 1995 wasn't far behind, and his wet-weather driving was second to none.
There's only one driver some consider a great that I flatly don't; Nelson Piquet, and he managed to win the title three times. He too didn't often have especially strong teammates, but circumstance and often bad luck on the part of others led him to three titles. Even Mansell generally had the beating of him, someone who despite often showing Alonso-levels of determination, few would consider a top driver.
