I'd say the worst Stuarts are Anne and Charles I. Anne was a selfish, foolish woman who spent 13 years at war over an issue that had no significance for England whatsoever. What did England care who sat on Spain's throne? As long as English trade continued uninterrupted it didn't matter. Not to mention she was a huge religious bigot against everything not Anglican and allowed the throne to pass to foreigners.
Charles I, while he wasn't attempting anything new or different (most of the continent was transitioning to absolute monarchies), went about it in completely wrong ways. He backed down over ship money after it had been declared legal by the courts, he backed down in both Bishops' wars with only one battle fought, acted WAY to arrogant for a man in his position during and after the Civil wars, basically gave Parliament the right to execute his ministers and got the monarchy overthrown. Sure he inherited a bad situation from James I, but he made it 10 times worse. A more conciliatory or strategic monarch could have succeeded in transforming England into an absolute monarchy. That he failed is a sign of weakness on his part.