You left out Calvin Coolidge.
Coolidge was great because he didn't have an agenda other than governing, vetoed almost every bill he saw and made a concerted effort to stay out of the news.
"Silent Cal"
Wouldn't that be a blessed relief?
Yup. Of course it all depends on how you define “greatness.”
To my mind it requires facing and defeating enormous obstacles. Coolidge might very well have had the potential to be as great as the other three, but never got the opportunity to demonstrate it, since nothing much happened while he was president.
Washington of course faced the need to literally invent a new nation, of a type that had never existed before. Lincoln faced the greatest of all existential threats to this nation. Reagan faced and defeated an ideology that most assumed was on its way to eventual victory.
I don’t, oddly enough, consider WWII to be a comparable challenge. As Churchill said after Hitler (idiotically) chose to declare war on the USA. “It’s all over but the shouting. (Paraphrasing.)
With the US in the war, the disparity in resources was just too great.