Chelsea huh?
She seems harder to unlock than some other girls, but rather easy to get afterwards. Apparently there isn't a gift that she won't love. Even weeds.
I'm thinking Sabrina might be the hardest. She's obtainable after shipping a lot of ore. To me, mining is very difficult in this game. One of the more difficult tasks in the game. Perhaps her.
Though, I think I'll aim for chelsea anyway. Then I'll go through all the events to have the other girls marry their respective others and keep them all in town, rather than them moving out when I ignore them.