It really depends on what you are trying to achieve, and how much infrastructure you want in place already, plus the ease with which you can find samples that help you out. I’m going to declare an interest here, because I’ve been actively involved in at least one MVVM framework, and I’ve had input into others through the WPF Disciples group, so I’m a little bit biased. Saying that, here goes:
Microsofts MVVM Toolkit – this is still very much in the alpha stages. When it was originally released, it took a bit of a savaging from the Disciples because of what it didn’t do. Saying that, MS is looking to beef this framework up so it’s one to watch – it’s just not ready yet.
MVVM Foundation – ah Josh Smith’s version of the framework. Josh is one of the daddies of MVVM, and has been a huge advocate and teacher of the pattern. As a result, a lot of what you’ll find in other frameworks has Josh’s fingerprints all over it. This framework is intended to provide the basics of MVVM, and not to address some of the more esoteric issues. Originally this was intended only for WPF, but people such as Laurent Bugnion and myself have added functionality/projects that mean this will be a Silverlight compatible framework as well.
WAF – no experience of it, so I can’t comment on it I’m afraid.
MVVM Light – Laurent Bugnion’s take on it, and just updated to version 2. This is a very good framework, but again it’s not intended to cover every single aspect of MVVM applications. Given Laurent’s background, it has very strong Silverlight and Blendability support in it.
Update Laurent has just informed me that the .NET 3.5 and .NET 4.0 versions are feature compatible. Wau to go Laurent.
Cinch – Sacha Barber’s excellent WPF only MVVM framework. This covers more ground than the frameworks I’ve talked about above. It’s an excellent framework, and takes advantage of concepts covered in Bill Kempf’s excellent Onyx project. Onyx is intended to complement MVVM frameworks, and adds in functionality that’s typically been hard for people to do in MVVM/WPF. Again, originally intended to be WPF only, Onyx has progressed to include SL compatibility – work I am particularly proud to have been involved in.
Prism – Again, I’ve never used it, but I’ve heard a lot of good things about it.
Ocean – Karl Shifflett, Program Manager on the Cider team, recently released a fully featured WPF MVVM framework. Again, this is an excellent framework and has lots to recommend it.
The bottom line is, download the different frameworks take a look at them and work out which one fits most intuitively with the way you think and with your requirements. If you think that you might want to support Silverlight from the same codebase then the WPF only frameworks should be discounted.