For rock, I like Graham Maby who performs with Joe Jackson and Marshall Crenshaw. I believe he's toured with Graham Parker, too. Very solid rhythm, and his occasional solos are interesting, too.

For jazz, I like Ray Drummond a lot. Very versatile and always a driving force. A nice showcase album for him is 'Two of a Kind,' a duet with pianist John Hicks.

Additional favorites other than those mentioned in previous posts include Charlie Haden (e.g., 'Beyond the Missouri Sky' with Pat Metheny and most of his Quartet West albums are excellent), Marc Johnson (Bill Evans' last bassist whose solo albums are also very good), and Mario Pavone (Thomas Chapin's bassist who also heads his own ensembles, comparable to Mingus).