I agree that it depends largely on your style, but also on things like what your amp is like. I like to keep things as simple as possible, so i got an amp with good distorted and clean tones (Mesa DC 5) and now i only have 3 pedals, and one of them is a tuner! the other 2 are a dunlop classic wah and a boss chorus.
if you are limiting the number of pedals (which i think is a great idea, it keeps your tone more 'pure') then i believe most guitar sounds can be approximated by using:
a good distortion type pedal (if your amp doesn't cut it. if you have a good tube amp, i'd be inclined to use the amp distortion)
a good wah
Chorus (you can get some resonable tremello effects out of a good one)
Delay
Flanger (this can do what a phaser will, not exactly the same, but close enough)
and always leave space for a decent tuner. doesn't matter how many funky sounds you can generate, if your guitar's not in tune, you will sound crap...
obviously if you a re looking for a specific effect (harmonising, ring modulation etc) then you need a specific unit to do that, but the above list shold enabvle you to get enough different sounds out of your rig to satisfy most styles and keep free form jams interesting...