I think my worst purchace may have been Gigastudio Solo - I never use it, it doesn't interface very well with Sonar. I originally bought the original Gigasampler for piano samples, and I bought the newer version for the same reason a couple of years ago, but it turns out all the piano sounds I use don't need it - they either come with their own "player" like some version of a NI player (like the Art Vista Virtual Grand Piano) or aren't samples at all, like TruePianos, which was one of my best purchaces.
Another thing I bought and never use (a common theme for me) is the Korg Padkontrol. It's a cool product for sure and most people seem to love it, but it turns out I prefer playing drums with my regular old keyboard.
My best purchaces include Sonar and my Echoaudio Mia card which which I can't sing the praises of enough.