Well most every guitar player should have an overdrive or a distortion - like an Ibanez Ts808 (amazing) and a Boss DS-1 or something similiar

A wah is pretty much an essential, most people either have a Vox or a Crybaby

A delay can be a very useful and musical effect if you use it right, I would recommend a digital delay because of it's versatility. The Boss DD-6 or DD-9 (i think its 9?) are good, but I have an Electro Harmonix Stereo Memory Man with Hazarai and it's great. Analog delays are great too, but they can't do as much as digitals can, and quite frankly if people don't play guitar they can't tell the difference between how analog delays sound and digital delays sound.

Those are pretty much the essential pedals most guitar players have. You can also add things to that though. I have a Dallas Arbiter Fuzz Face, Dunlop Octavia, Dunlop Univibe, and I'm going to get a Digitech Whammy sometime. But if you're going to invest in pedals get a good digital delay, overdrive/distortion, and a wah.
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