What is the most common design patterns for any windows forms application?

Jeremy Miller's guide is an excellent starting place. It covers several important patterns including:

• Model - View - Controller - There are several flavours, make sure you examine them all
• Automated unit testing of GUI binding code
• Command pattern
• Registry

You will also want to look at inversion of control and dependency inversion. Fowler's overview does a good job of explaining the concept. When you've looked at this, here is a tutorial on just one implementation of an IoC tool.

If you are still hungry for more, check out Rich Newman's series on Microsoft's Composite Application Block. The CAB is chock full of patterns, but avoid the official documentation as it is notoriously di