What is the best implementation for AOP in .Net?

I think that Castle Dynamic Proxy is the solution of choice if dynamic interception can handle your needs. This framework is used internally by a lot of other frameworks that want to offer AOP capabilities. Typically, most of existing IoC containers now provide some dynamic interception mechanisms (Spring.NET, Castle Windsor, StructureMap, etc.) If you already work with an IoC container, maybe it could be easier to look at what it proposes.

If dynamic interception can't address your needs (weaving sealed class, intercepting non-virtual call, etc.), then you certainly want static weaving. PostSharp is the reference in this domain.

Note that it also exists Linfu, that can be used to leverage both AOP fashions.

"Best" is subjective.

First, draw up a list of the features you need, your architecture, etc. Then look for options that do what you need, without introducing unnecessary complexity. For example, several are interface oriented: is your code currently interface oriented? If not, PostSharp might be a better choice (being weaved into the original classes). But of course, PostSharp can't be configured at runtime... horses for courses.