This depends on whether we are trying to judge how effective they were or how good they were.
If we are trying to judge how effective they were, we should look at the extent to which they achieved what they hoped to achieve. We can look at Franklin Roosevelt, for example, and say that he was effective because he got his New Deal programs passed and because he led the US through WWII.
If we want to judge how good they were, we need to look also at whether we think what they were trying to accomplish was beneficial. Again looking at Roosevelt, conservative historians might argue that he was not a great president because what he was trying to achieve was the creation of a big government welfare state that, to conservatives, has harmed America in the long run.