Basically 100% agree with this.damian13ster said:I agree with him. For a cyclist that already won all GTs, winning a monument is much more impressive than continue amassing Giros and Vueltas.
So Nibali might not be the best rider purely in GTs, but if Boonen classifies him as a GT rider, then he is the best among cyclists classified as GT riders (if you can follow that logic).
I think it is between him and Valverde as best cyclist of this generation, but since Valverde is stained by a clinic ban, I think the title goes to Nibali. If only one of them could win a WC, that would pretty much settle the discussion