Honolulu, but I don't think he deserves one.
Presidential libraries are something to be "deserved" by special circumstances now? Not just being president isn't enough?
Chicago is the obvious choice due to the reasons Grumps listed. Anything else would be dumb. George W. Bush's library isn't in Connecticut...
On the other hand, Hoover's is in Iowa and not California.
Fair enough, although I do tend to identify those two with those respective states rather than California (actually, admittedly I forgot they ran with CA/NY as their "home states"). Maybe it's just me but I identify Obama more with Chicago than Hawaii.
Besides all that, there are reasons enough that it should be in Chicago rather than HI, especially if they want... you know... people to actually go to it...
A good question to ask is where would a potential Hillary Clinton library be? Obama has plenty of ties that bind him strongly to Chicago. Hillary has always kind of floated around. Besides her Senate campaign (which was pretty strong carpetbagging), does she really have strong ties to New York state?
Poll
