Day One is one of the best journaling apps that is minimalistic, has a nice looking user interface.

It keeps the notes / journals datewise which is important if you want that particular functionality as very few journaling app lets you do that.

Yo uan export your entire journal into a PDF format in matter of few clicks. 

If you wish to keep a separate that keeps a log of your daily musings, happens, notes. You can explore Day One app.