We use the comparative to compare one person, thing, action, event or group with another person, thing etc. We use the superlative to compare somebody/something with the whole group that she/he/it belongs to.
- Mary is taller than her three sisters.
- Mary is the tallest of the four girls.
- He plays better than anybody else in the team.
- He is the best player in the team.
- Iron is more useful than any other metal.
- Iron is the most useful metal.
When a group only has two members, we sometimes use the comparative instead of the superlative.
- John and Tom are clever boys, but I think John is the cleverer/cleverest of the two.
- Take the shorter/shortest of the two routes.
- Alice and Mary are rich women. But Alice is the richer/richest of the two.
Some people feel that a superlative is incorrect in this case.