The "panned phaser" model on the Line 6 MM4 (based on Ibanez Flying Pan) is the thing that impressed me the most about that pedal. In stereo it made me dizzy.
I think the Marshall Vibratrem deserves a mention here. It can be run with tremolo on one output and vibrato on the other for one of the most beautiful shimmering guitar sounds I have ever heard. It might not be the best sounding trem around (although it isn't bad by any means), but running tremolo/vibrato in stereo is a jaw dropper.
The panning on my Looker tremolo is amazing. I can get some cool, slow ping pong effects. Or if I set it real fast, I get sweet leslie-ish vibes going.