Stupid poll.
First, Kennedy was an excellent statesman who actually consulted and listened to Eisenhower, and he matured steadily throughout his short Presidency. He was deeply flawed, but one of our great historic Presidents. It's fairly easy to see that with almost anyone else at the helm, the CMC might have become WW3.
Second, Carter was not NEAR as bad as the naysayers like to say. He inherited a shitty economy and wobbly foreign situations, and did little to either better them or worsen them. Blaming Carter for the crappy economy of the time is like blaming Bush for the post-9/11 dip. It's unrealistic at best, and pure political opportunism at worst (ie; I'm a Republican and I want to sh*t on Carter because he was a Dem). Carter had a remarkable Navy career, and has worked tirelessly for humanitarian causes for decades. I'd honestly take Carter over fuckwads like GWB or LBJ any day of the week, month, year, or lifetime.
Obama will probably be much different than anyone who's come before, partly by internal personal reasons (things that make Obama unique), and partly by external reasons (mood of the country, the opposing party's willingness to compromise or not, foreign situations that need attention)
We won't really know until (A)- He's the Dem candidate, (B)- He beats McCain, and (C)- He proves his relative worth during his term(s).