My worst puchase was cardstock from Michaels. I really liked it until I got my first order from SU. I didn't want to waste it, so I cut it up into half sheets and quarter sheets along with some Wal-mart white card stock and made each of my nieces their own stamping tote. Complete with a basic black stamp pad, markers, watercolored pencils, a blender pen, some envelopes and tons of stamps. They love them and use them all the time. Now I can justify buying more paper to replace it.
My best purchase was probably my stamp-a-ma-jig. I am a little anal about my centering and lining everything up, so my SAMJ has been well used and is well loved.