Assuming you are talking about UML diagrams, I have not worked professionally with anyone, but I find that different diagrams work well for different diagrams.
I prefer ArgoUML for class diagrams, UMLet sequence diagrams for deployment, and component diagrams Activity diagram diagram
This covers the charts that I spend most of my time.
I find that UMLet and Dia are easy to use and have collected almost all the diagrams I need, and ArgoUML has additional functions that help with specific diagrams, and also provides code generation from class diagrams.
I also used Borland Together, and its robin round functions are good, it was very difficult for me to work if you did not use it as Borland assumed, which often becomes disappointing.
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