A lot to choose from! Which country should you collect? There are as many answers as countries, so start by collecting a country that has special meaning for you or one that has lots of colorful, yet affordable. Countries having various topical items may appeal to your taste and can include boats, trains, animals, people, allegorical scenes, birds, values such as 1000 Yen and more, can be found. Collecting foreign paper is both enjoyable and educational at the same time.
I recommend the "Standard Catalog of World Paper Money General Issues 1368-1960, 13th edition. Every foreign paper money collector should use this as a reference along with "Standard Catalog of World Paper Money - Modern Issues: 1961-Present."