I never really got into Slash's solo stuff, but his work with GnR is immense. He's got that 'singing' tone, has an incredible taste for musicality and melody. the musical range of the "Use Your Illusion" albums and the creativity that the whole band put into it is nothing short of amazing - you find anything from an acoustic blues to metal and punk. It was probably GnR that brought out the best in him, but for that little oeuvre alone he deserves all the recognition he gets.
Plus, he does look really cool.