I've heard others state that "Another Thin Man" is good but not *as* good as the first two Thin Man films. Some of this may be because this film is in many ways different from the first two and as a result may fail to meet the expectations of some who view the films in order.
Some individual elements of "The Thin Man" may be stronger. However, as a stand-alone entity "Another Thin Man" it is the most entertaining film in the series and arguably has the most depth. This film succeeds with its dialog and its visuals, with its humor and its intrigue, and Powell and Loy are at their bantering best.
Watching the previous films is in no way a prerequisite for understanding and enjoying "Another Thin Man." If you've never seen a Thin Man feature, watch this one first.