What makes a penny valuable?

Generally, coins are valuable because of the price of the metals they are made from, but if you mean in a collectible way:

One thing that makes a penny valuable is the date. They older the coin usually the more valuable. Another thing is the mintmark. For example 1943 pennies with a "S" mintmark are more valuable than pennies with no mintmark. Another thing is the condition of the coin. The less ware it has the more it's worth.