I really like Muji pens because of their no-nonsense, minimalist design.  The pens themselves are gel pens with a really precise writing tip, so they are good writing instruments on their own.  But they are also extremely desirable due to their non-branded, clear plastic casing.  You can instantly see which color ink you are using and how much you have left, and the translucent plastic gives the pens a simple yet elegant and not cheap looking aesthetic.  I also love how they have their tip point value on the top of the cap so you can easily see which pen you are selecting.