Altogether, old Guns and new Guns, nobody can outshine Slash in my book (Izzy is a close second).
From the new Guns, I think Buckethead would be the best for writing new music though I've always loved Robin the most.
The actual current line-up, DJ Ashba has the most potential to become a very recognizable figure in the Guns N' Roses context but my favourite would probably be Tommy Stinson.