Mick himself has appeared on the series a number of times (most recently in the season finale), and other band members have been ripped on in the past (see Mike Myers' Keith Richards).  But the best moment was when Jimmy Fallon played Mick, accross from Mick, as a sort of voice the the mirror.  Seeing two go back and forth, and seeing a humble glint in Mick's eye, was classic.  And for an impression like that, more than just the voice, there are all the hand gestures and various little stage-fidgets.  Fallon certainly lives up to the position of professional mirror.