Try https://www.lucidchart.com . It is slick, has real-time collaboration, has UML (including via markup) and lots of other diagrams. Freemium that allows you to do most things.
Unfortunately lucidchart is only a drawing tool (no model validation, no documentation nor code generation).
If you need both real-time collaboration and real UML tool, you should try https://www.genmymodel.com