Noun, verb, adjective or adverb?

His back was nicely tanned.

The car backed up and hit the tree.

My father is searching for the back issues of the magazine.

After the divorce, she went back to her parents’ house.

They did their best but still couldn't beat the other team.

The goal was to best the competition in any way possible.

She said The Lord of the Rings is the best movie of all time.

He played best after a couple of martinis.

He has decided that a change for the better has to happen now.

Amazingly she bettered her own record in the shortest time.