Which is the most sought after US coin?

If you're referring to U.S. coins it's a race between the quarter and the penny.

Because the Mint made a laundry list of mistakes back in the 1960s that led to the near-disappearance of half dollars, the quarter has taken on the role of the most-used coin for larger amounts of change-making. Before 1965 any purchase that required more than 50 cents in change would be made with a half dollar, and possibly one quarter; now it's 3 quarters.

OTOH although it's not very popular the penny is probably the most frequently used for two reasons: First, the presence of sales taxes and "99 cent" pricing in the U.S. means that most purchases rarely end up in exact multiples of 5 or 10 cents. Second, the failure of the U.S. to adopt 2-cent coins (compare to the U.K. and the EU) means up to four pennies are needed for making change, the most of any denomination.