The thing that finally convinced me that Ferrell could act wasn't Stranger Than Fiction--though he was fine in that--it was The Producers. He was still playing for over-the-top comedy, but it was different from his normal style; he was playing an actual role, and doing it (imo) pretty well.
Uh, everyone's said Keanu Reeves (and they're right), so I'll pick... Heather Graham, the Megan Fox of a decade or so ago.