Looking for advice - Dependency Injection over Service Locator in Mef
Service locator is an anti-pattern because it tends to create implicit dependencies which are not easy to see and usually cause grieve when trying to write unit tests. Especially your implementation which references what seems like a singleton instance which can cause additional problems (i.e. you need to take care to reset it between tests).
Avoid it.

I can't say much about the MEF injection but it seems roundabout having to write all that code. It also creates an implicit dependency which will probably cause grieve when unit testing your class and makes it directly dependent on MEF while there is no need for it.
Avoid it.

Clean and simple. Explicit dependency, easy to unit test and injection can be easily manged by any IoC container of your choice (Unity, Ninject, Windsor, etc.).
The way to go.