Best: The Stamp-A-Ma-Jig! I resisted buying one for sooo long because I just didn't understand how it worked. Then the clerk at my LSS explained it to me and a lightbulb went on. I stamp a lot with Rosie's Roadshow stamps, which require lots of masking and exact placement. The SMJ made a huge difference.
Worst: A set of CTMH My Acrylix that I bought at a stamp fair last year. Tried using them once--they don't stamp that well--and it is not like I'm anti-clear stamp or anything. Just can't seem to get these ones to work for me.
-Molly, Stamping for her Sanity in Seattle