I don't think Djokovic will get to 24 as I think Rusedski said last week, but he will end up with more slams than Federer and Nadal, and people will justifiably call him the greatest. I personally believe that what we have seen with all of them - though Djokovic and Nadal have benefitted more than Federer - is the impact of consolidating and general slowing of court speeds - alongside developments in racket/string technology, of course. The sport has moved in a direction that has enabled the strong baseliners to dominate in a way that just wasn't possible previously: you barely need to adapt for different surfaces now, which I think is a huge shame.
As amazing as all three are, would they have won 50% more slams than Sampras playing with that same variety of court speed, and - given there were only 16 seeds - the potential to face a huge serving 17th ranked grass court specialist in the first round at Wimbledon? I just don't think there's a chance of that. I do think Federer would be the best equipped to succeed in that alternate reality, but clearly Djokovic is perfectly suited to the one we actually live in. Djokovic fans would I'm sure dismiss this as the ramblings of a sad Federer fan, which maybe isn't far from the truth.