Billy's Electro Harmonix Polyphase, from the "Stompland" video
Considered by many one of the greatest phaser, the Polyphase has some unique features as well as a unique tone. It features an eight stage optical phase shifter (although six stages are variable), a very complex LFO and, the most important feature, an envelope follower: the phase shift can be either controlled by the internal LFO or by the input signal strength (envelope), allowing the musician an incredible number of different tonal results.
[product page (reissue)]
Featured in a video of Billy's huge collection of guitar pedals called Stompland.