Since we already had a license for the Infragistics suite, we went with their Silverlight "Geographic Map Control".
I would not recommend this (Infragistics): While writing a very basic POC, I encountered 1 serious visual bug (since fixed) + one piece of code had to be rewritten in a very specific way (THEIR way) because my alternative solutions were all extremely slow.
I have not checked the XAML Map Control Clemens suggested (a project he is involved with) because we needed offline map in Silverlight aswell.