I think the Dunlop crybaby is perfect for fat, thick tone. The classic wah pedal, and you can usually pick one up used for pretty cheap. The vox ones are similar but not as good , I think-- a little fizzier and thinner-sounding to my ears.
The DOD ones are also cool in a different way, sort of cleaner and clearer-sounding-- less whiny and more sort of plinky and warbly-- more like a wow-wow than a wah-wah, if that makes any sense. Good for clean-tone funk rythm stuff, like a porno soundtrack.
The morley ones are very extreme-- very nasally and weak-sounding at extreme settings. If your footwork with the pedal is skillful and controlled, you can get an awesome range of tones out of it, but it takes practice. They are harder and less intuitive to use than the cry babys, I think, but a skillful operator can probably get a massive range of sound variations.
But of course the best way to pick one is to take your guitar down to your local music store and try a few of them and see which one does the best job of getting you where you need to go.
A word to the wise-- most of the "Artist signature" models are things that basically charge an extra $30 to have a rock star's name attached to a variant pedal, when the rock star in question actually got famous using the original base model. Like Johnny Rocker records a hit record using a dunlop crybaby, and dunlop comes up with a "Johnny Rocker" model after the fact that has some special feature or paint job, and charges extra for it. I'm not saying the Johnny Rocker is bad, just that chances are you can get the same sound he got on whatever record made him famous enough to warrant a special model by using the stock wah pedal.
Cheers.