I've used a flanger (it's kind of a souped-up chorus pedal) and gotten some 'way-out-there results. Sounded like aliens were landing! Also used a fuzz pedal (feeds back too much for my taste), digital delay (lots of fun), an equalizer pedal (millions of uses), and an octave pedal (turns me into a nasty horn section).
If I could only have one, I'd use an equalizer -- that way, I can separate my solo sound from my "section" playing with a good stomp. If yours has a volume control on it, so much the better. But I love using that octave pedal, too.
Problem with some pedals is if you're bending (which I do a LOT) then it messes up the signal. I notice I lose my bottom with the octave pedal on an down-and-up bend. I assume if you're using a pitch shifter (which I'd still like to try) you'll also lose it when you bend.
Story tying in with that octave pedal: soon after I got it, I was giggin' in California with a three-piece -- piano, bass, harp. The bass player got a little too high and perhaps he thought time went faster than it did. He packed up and left during the second set. Down to just me and the piano. I went to that octave pedal a lot more after he left, just to give us some "bottom." Wasn't pretty, but it worked.