There is an increased demand for $20 bills and $100 dollar bills. $100 dollar bills, as I understand, are printed slightly differently for overseas circulation and at-home spending.
My guess would be the $1 bill, since it gets the most circulation and wear and tear.

http://www.moneyfactory.gov/uscurrency/annualproductionfigures.html

THe answer to me is shocking.

Its $100. Interesting site. Shows printing of all the denominations for the last 3 decades. Something is not right if this country is printing more $100s than $1′s.

I’m no economist, however, my guess would be that the $100, because of its denomination size, doesn’t see frequent enough circulation to get it back into the supply. Banks, if I’m not mistaken will retire certain bills and get new ones from the fed. Then the old ones returned to the fed are destroyed. But only banks do this. And since most retailers don’t accept anything larger than a $20 and most large purchases (bills, groceries, etc) are charged to check or plastic, the $100 bill becomes a piece of paper that we have to go to the bank to break into $20′s. The $50 dollar bill is equally as useless since there are plenty of 20′s and 10′s in circulation. $100 bills seem to disappear out of circulation, I suspect, because the end up collecting dust in bank vaults, never seeing an actual register. My question is, why recirculate a pointless bill? For Vegas’s sake?

Most people that would have $100 to their name just swipe their credit card for anything worth that much. Cash is what you put in a vending machine, and even vending machines are starting to go plastic. Only drug dealers and prostitutes (well, maybe not the prostitutes in Bakersfield-they aint worth $20) carry with them paper 50′s and 100s anymore.