I going with Exile. Even though his vocals aren't always pitch perfect, for my money, Keith is the best back-up vovalist ever.....it's a weird timing thing I guess.....Mick backing Keith on Happy, a slew of other backup singers and lead vocals that are buried in the mix, giving prominence to the backup vox. My opinion.
I actually think - the stones would be better without Bernard and Lisa. I realize that in their best recordings - backing vocals were done by the band members themselves. I think Lisa, Chuck and other extra musicians on stage are the reason the Stones are turning into Las Vegas act.