Dean Martin:
SOME CAME RUNNING
ROBIN AND THE SEVEN HOODS
I wish I could add BELLS ARE RINGING. He's absolutely perfectly cast, and his voice is just wonderful in the singing. But in the actual film, he's BARELY trying at all. In the "I Met a Girl" number, he doesn't even bother to open his mouth to lip-synch during the playback. (He actually appears to be drunk while fighting his way through those crowds.)
I know there are a lot of Jerry Lewis fans on the site, but Martin is the only reason I can make it through the Martin & Lewis comedies now. I just find Jerry so oppressively "on" all the time, and I lose patience with his characters within ten minutes of his first appearance in those movies.