Through the last few years, building systems became very complicated; any complicated system needs a tool to be built. Thus choosing the best tool needs to know the most important features of it. Today UML is considered as a de facto standard in software development and is used in many domains ranging from scientific modeling to business modeling. This paper describes the most used UML tools; it defines the important features of each one then compares between them according to their features, and it also defines the criteria for choosing the best tool in building systems