4. Boss OC-3 Super Octave
For me, Boss are at their best when they are producing pedals that process your signal and turn it into something completely different. The DD-7, the OC-3, the Tera Echo, the Harmonist. They are all incredible fun, and really make you play in a different way.
The Boss OC-3 Super Octave has one function - it makes your guitar sound incredibly evil by adding in an extra octave (or two) up to two octaves below your input signal.
The OC-3 also includes a drive mode, for adding distortion to the output signal, and also a polyphonic octave mode, so you can play chords.
It just sounds great, and it inspires some pretty cool riffs.