IMHO:

Worth considerably more than nominal value: $1 bills, Quarters (if you have any coin-op machines in your life, else slightly less than nominal value)

Worth slightly more than nominal value: $5, $10, $20 bills (also, all increasingly valuable as number of $1 bills increases)

Worth slightly less than nominal value: Half-dollars, Dollar coins, $2 bills (unless you're sentimental about unusual currency)

Worth considerably less than nominal value: $50, $100 bills

Worthless: Pennies, Nickels, Dimes