Those answers (especially Hanks and Streep) are more suited for "which actor/actress ain't your cup of tea", as opposed to "worst actor/actress" which is just silly in the case of those two (likewise Harrison Ford who one person mentioned). The wooden Emma Watson is definitely suited to the latter topic.
The best assessment of acting that I've heard was made by an actor in reference to the exceptional Daniel Day-Lewis: the vast majority of actors and actresses are disposable in that you could easily replace them in a film and the film would be unaffected (bascially they were fortunate to get the gig). It's the unique quality an actor/actress brings that really sets them apart. Schwarzenegger clearly wont be winning any prizes for his acting, but he has a unique presence and charisma that makes him irreplaceable. Likewise you couldn't replace Rickman, A.Hopkins, Day-Lewis, J.Andrews, Depp, Hanks, Connery, Grant, Cruise etc. Denzel Washington has been in quite a few weak films (poor script), but the unique quality he brings makes them more watchable than they otherwise would be. Back to the Future would not have been the same classic that it is had Eric Stolz played Marty McFly and not Michael J. Fox. Emma Thompson (great actress in her own right) once said A.Hepburn was not a good actress, to which a son of Hepburn's brilliantly responded, "she may not have been, but she was a star".