You are invited to make your own nominations in the comments, with one stipulation: the incumbent is not eligible for any of these, because 1) I think we should judge the entirety of the term(s), and 2) aren't you just a little tired of talking about him?
I've turned this over in my head for years, and I always come back to the same answer: George Washington. He set every precedent that counts, from eschewing any title more ostentatious than "Mr. President" to gracefully ducking out after two terms. This country was very fortunate to have a fan of Cincinnatus as its first executive.
George W. Bush is certainly my generation's Richard Nixon, but Richard Nixon was Richard Nixon. The man might very well have committed treason while running for president in 1968 (and likely lengthened the Vietnam War in doing so), and it really kind of went downhill from there.
Most Underrated President
This is difficult; there are a lot of presidents I (and most people) just don't know very much about, which almost certainly makes several of them more literally underrated than George H.W. Bush. Still, we're talking about a man who ushered the U.S. (and really, an awful lot of the world) more or less safely through the collapse of communism, and assembled a truly broad international coalition to contain Saddam Hussien. He wasn't perfect--and I certainly have no love for the Bush family--but can you seriously imagine Mike Dukakis pulling all that off? I can't.
I just wish I could go back in time to 1945 and talk him into getting a vasectomy.
Most Overrated President
In my bones I want to say Reagan, but seriously, what is that thing you Boomers have for John F. Kennedy? He averted a nuclear war only because he damn near started one. (And don't try to tell me the Soviets started it by moving missiles onto the island to repel a U.S. invasion, because a U.S. invasion of Cuba wasn't exactly a paranoid fantasy, was it?) If he hadn't been martyred, we'd blame him for Vietnam, too.
Most Complicated President
Being born in a city that was burned to the ground by Union forces during the Civil War probably has more than a little to do with it, but I don't know how anyone could answer this with anyone other than Abraham Lincoln. Ending slavery? Awesome. Prosecuting a war that killed approximately 2% of the U.S. population? Less so. Regardless of what you think of Lincoln: we are still living the legacy of the Civil War in our politics today. A century and a half later.
Would it have been better to simply let the South go? Would doing so have ultimately threatened the security of the remaining United States? I have no idea, and neither do you. Like I said...it's complicated.