Overall body of work, I think Dwight Yoakum plays an awesome scumbag/villian. You might find some good answers here in my 'best movie role by a musician' thread: http://bbs.clutchfans.net/showthread.php?t=141524&highlight=musician
Talking about a lot of guys, and that's fair, but I believe there is one singer/musician who has settled the argument already. Cher. Can't argue with the Oscar. None of those other people mentioned have one, nor will they.
ummm, wrong. Frank Sinatre has an Oscar for From Here to Eternity (Supporting Role), and he was nominating for Best Actor in "The Man with the Golden Arm". Plus, the only reason he didn't win anything for his best movie and role in Manchurian Candidate was because the studio pulled it before release because it was so controversial. plus, he has several Emmy's and Golden Globes. Cher? bah!
Agree about Sinatra. The guy was an excellent actor. This fellow isn't bad, either, although not in the same class...
I give my vote to Mark Wahlberg, who is one of my favorite actors working. Honorable mention: Ludacris is the best hip-hop artist at playing a hip-hop artist.