Not only vocals. A great bass line or drum pattern gets me.

Example:

Alex Ligertwood: vocals on "Everything Must Change" (Steve Ferrone) and "Brighter Star" (Santana)
Neil Stubenhaus: bass on "Out Of The Night" and "Killer Joe" with Quincy Jones amonst tons of other things I love from him.
Ron Wikso: drums on for instance "Best I Can Be" off Fergie Frederiksen's "Equilibrium" and "Sing For Me Brother" (David Glen Eisley)
Michael O'Neill: FABULOUS guitar solo on "Georgy Porgy" off Mike Porcaro's "Brotherly Love"
David Coverdale on most of Whitesnake's song from 1978/1981.
Vivian Campbell's solo on "Don't Talk To Strangers"
Gary Moore's many version of "parisienne Walkways"
David Garfield's soli on "Nothing Personal" (Karizma - Document) and "Zuke" ("Giving Back" solo album)

Those are a few...