What is the rarest 2 dollar bill?

As of 08/2011 the highest auction prices are for an 1869 United States Note. Circulated ones go for $500 to $1200 depending on condition, while uncirculated ones are around $4000.

FWIW, there are no truly rare modern (i.e. 1928 and later) $2 bills. The only one that cracks 4 figures is the 1928 B issue which sells for about $1000 in uncirculated condition.