It's a pretty accurate list, notwithstanding your inclusion of Blake and Gonzalez, which frankly, is laughable. I would also argue that Ferrero deserves to be included in it. And Agassi was still going strong when Federer burst on to the scene (he won the Austrlian Open the year Federer rose to prominence and lost a memorable US Open final against him in 2005), and didn't retire until 2006 (when he was still making Masters finals every year).
You're also forgetting that it was Hewitt who knocked Sampras off his perch, and stole his number 1 spot from him. This while Sampras was still very much a force to be reckoned with (as evidenced by his US open win in 2002). It was a turning point for the game of tennis, as far as I'm concerned.
And with regards to Federer, if Nadal is the 'only credible challenge who came around', (incidentally the same Nadal who is now being touted as Federer's natural sucessor,) then it's hardly a stick to beat him with, is it? The greatest player who ever lived is now being succeeded by, quite possibly, the one who will be considered the greatest in the future.
Nadal was hopelessly outclassed and outmuscled in his early encounters with Federer, and to his credit, has adapted his game, and his physique, to overcome that. He is one of the most dedicated sportsmen on the planet, and he's had to be to get where he is. Again, I think you're underestimating Nadal's achievements in the same way you have Federer's.
But like I said, this debate will always split the room, so it's probably not worth labouring on.