My encounter with all this liberating knowledge and inquisitive atmosphere [at Cambridge] was very nearly over before it had begun. In my very first term, in October 1962, President Kennedy went to the brink, as the saying invariably goes, over Cuba. I shall never forget where I was standing and what I was doing on the day he nearly killed me. (p.65)If I interpret correctly, this event would result in the political Hitchens we know today (p.66). Thanks to Kennedy, you and I have Vasili Arkhipov to thank for the existence of civilization.
Whether or the lead-up and content of this crisis - as flanked by that whole Vietnam thing - make Kennedy `the worst', he certainly has his hat placed well within the ring. For the 20th century, I would also nominate Wilson, Nixon, Reagan, and Clinton - especially Wilson and Reagan. For this century, I cannot yet decide whether or not Obama will turn out to be better than Bush. (For the issues most important to me, they are extremely similar.) There are only two options for the 18th century as well, but the signing of the Alien and Sedition Acts suggests Adams. It's difficult to make a judgement on the 19th century: should we pick the worst as regards slavery, dooming us to a future civil war? The one who was most genocidal with respect to the Native Americans? What about the corruptions of the Gilded age?
Here I don't know enough for any strong opinion. Millard Filmore is up there, along with Ulysses S. Grant.
I would say that Lincoln is overrated, but he's nowhere near the worst. Unfortunately, people tend to think highly of Wilson, Kennedy, and Reagan. And boy are those some doozies. As far as justified favorites, FDR is a good choice. Teddy Roosevelt is very mixed bag, so I tend to think him overrated as well. But, like Lincoln, he is at least better than most.
It's more difficult to think of underrated presidents. This might be my cynical side, but perhaps it's more important to deflate inflated reputations than inflate deflated ones.