What is the most valuable year for a US nickel?

There are no regular-circulation nickels that are spectacularly valuable. Some of the more expensive ones are (prices as of 07/2009 for uncirculated coins graded MS60)

1880 shield design - about $5000

1885 Liberty head - $2500

1924-S Indian Head - $2600

The really valuable ones are errors or otherwise unusual. The top of course is the 1913 Liberty design of which only 5 are known. These were not authorized pieces and are suspected to have been minted after-hours by employees who used dies that were supposed to have been destroyed when the Mint decided to move up the striking date for the new Indian head coins. Auction values have been in excess of $5 million.

The other coins are errors:

1916 with a recut date, resulting in a shadow "16" under the regular digits, around $54,000

1918-D with a recut date from 1917, around $28,000

Several other dates are in the 2 to 3 thousand range.