I would say Will Smith, Frank Sinatra and Elvis are the top Musician/Actors
If we consider people who are mostly just pure singers (meaning not known for being a songwriter or for playing an instrument for anything other than mostly show) -- and I can't tell you how much I hate to say this -- Barbra Streisand is way, way ahead of everybody else. Two Oscars, eight Grammys, four Emmys.
I would agree with Streisand, Will Smith, Sinatra and Elvis.
However, I have not seen one movie with Streisand. When was the last time she made a movie? Also, aren't Emmys for TV? Was she on a TV show before becoming a hollywood legend?
What movies has she made besides the 2 she won oscars for, that are known?
I'd have to go with Will Smith hands down for the last 30 years, before that I'm to young to really know about Elvis, Sinatra, etc.