FWIW, I never thought any of them (Fisher, Hamil, Ford) were very good actors. But what they did have is on-screen chemistry - they weren't great individually, but they were fantastic together. It takes more than a good script to achieve that.
Objectively, the only really great actors in the OT, in my opinion, were Guiness and Cushing. Every time they were on screen they lit it up. And Yoda, but I'm not sure what that means.