Cool thread! Up-front disclaimer: All statements of preference are, of course, imho. :lol:
Mick Taylor was definitely the best replacement guitarist for the Stones; my personal favorite of any guitarist the Stones ever had or still have. Ron Wood is probably the best "fit" they ever had (grooves like the Keef, but with cooler chops), but he never played better than when he was with The Faces.
Steve Gaines - He wasn't really a replacement, but rather an addition to Rossington and Collins. And, by account of everybody in the band at that time, he was their best guitarist.
But my pick for best replacement guitarist has to be Marc Ford who replaced Jeff Cease after the first Black Crowes album. I liked Luther Dickinson and the new guy, Jackie Greene, but Marc really brought something special.