Mine would be:
1. The $4 bill (could be the common 1902 as long as I can get the best grade for my buck)
2. 1935 $25 bill
3. 1935 $20 bill
4. DOM. of Canada $5 (1912 or 1924)
5. 1935 $500 bill *
*For what it's worth and the attainability, if I was getting book price, I would rather spend my money on a great quality $20 then an "okay" quality $500 that already costs more than the cost of a new Ford Mustang + college tuition for my kids + down payment for condo.
The $20 and $25 bills are more desirable then the $500 bill in the same sense that the girl next door is more desirable than Carmen Electra or Angelina Jolie.
List yours...