If we can include people that never ran but should've, it would be: Eugene V. Debs, Norman Thomas, Robert M. La Follette, Henry A. Wallace, Wayne Morse, George McGovern, Ralph Nader, Paul Wellstone, Bernie Sanders, Hiram Johnson, and Barry Commoner. Not necessarily in that order. (though for me, Debs would still top the list).
Er, McGovern did run.
I didn't say that the list didn't include people who didn't run. Though actually, I have some revisions to make for the list. The new and improved version:
Top 5 best Presidents that never were: Eugene V. Debs, Robert La Follette, Bobby Kennedy, Bernie Sanders, George McGovern. (haven't figured out the order yet, and it's possible that I never will)
Honorable mentions: Norman Thomas, Ralph Nader, Paul Wellstone.