What is a collectible penny to look for? Nickel, dime, quarter, half dollar? The details matter.

I have a five gallon bucket that I have been throwing my American spare change into for about the last twenty years. (I leave my riyals, dinars and dirhams etc. behind)
I was about to take it to one of those counting machines and trade it in on currency, but it crossed my mind there might be something collectible in there.
Anybody know a penny, nickel, dime, quarter or half dollar I should be looking for?