The greatest is too hard. I think if Allman had of lived past 24 25 he would knock a few off their perch. Clapton is my favourite, and I really enjoy Knopfler and Gilmour's stuff. I love Brian May too. From my own personal preferences, I probably would go:
Allman
Clapton
Gilmour
Knopfler
May
Go the Brits! LOL!