According to "How UML Is Used," an article in the May 2006 issue of CACM, the UML diagrams that most commonly "provide new info" above-and-beyond use-case narratives are:

1. Class diagrams
2. Statechart diagrams
3. Sequence diagrams

Interestingly, "usage rates are not well explained by how much new information is provided." Statecharts, the 2nd most useful diagram, are used in most projects by only perhaps 1/4 of practitioners. Use-case diagrams, in comparison, are the 2nd most commonly used type of chart, but are one of the least effective in terms of adding value to use-case narratives (well, yeah...). Class diagrams are both the most useful and most used, while sequence diagrams are commonly used by about half the practitioners.

Rounding out the studied diagrams (they skipped Object, Component, and Deployment diagrams), Collaboration and Activity diagrams are, when used, considered useful by more than 60% of practitioners.