re: Has any actor ever ruined anything more?

I can't really say Leto was a terrible Joker. I just think The Joker will never be topped ever again after Heath Ledger. We can expect more Jared Leto as The Joker so he has his chance to further develop the character. With the little screen time he got in Suicide Squad* I don't think he could really get his moment to shine.
As for Eisenberg it's the same thing for me. I don't find him to fit the role but if you compare him to the Kevin Spacey version, Eisenberg blows him out of the water.

As for Deb in Dexter. The dialogue was never any good. That show took a major dive when the writer strike happened. The guy who played Dexter was the only quality actor on that show as well. Very disappointed with how Dexter went down hill. For a second looked like it could be the best ever.

And Star Wars prequels... I can't blame Christensen that was a weak effort from everybody.