Billy Joel and Elton John together get my vote but then I like James Taylor, too and I'm a major Bill Cosby, Billy Crystal and Robin Williams fan!
Years ago I saw Petula Clark and Jim Nabors (separate cruises) and they really put on great shows, in fact, they added shows but this was on longer cruises, not 7 days. Jim Nabors told some of the longest, funniest stories I've ever heard in my life. The theater was packed, SRO, and everyone was absolutely roaring and no one wanted the show to end. He put on the longest show I've ever seen and they had to repeat it several times.
I also saw Phyllis Diller in 1995 and poor thing was pitiful when I saw her getting off the next day in San Juan, trench coat, oxfords, wig with a scarf holding it on, the way she looked made me sad actually.