Dunlop 535Q Crybaby, Boss DD-6 Digital Delay, Electro Harmonix Big Muff USA, Digitech Whammy...

...are there any pedals I should add to this collection which you believe produce an essential effect or tone?
What kinda music do you play? What kind of sounds are you trying to achieve?

Without being harsh, upgrading your amp and guitar will help a whole lot more than buying loads of pedals.
+1. I'd get a general reverb mabye and call it quits on the pedals. If you get a new amp, and you choose a combo, and it has effects like most combos, you'll end up regretting some of those pedals...like I did. I sold all three of my distortoin boxes when I got a better amp then my 10 watt practice shite-box. (But your amp actually isn't that bad, I use one for recording)
Yeah, upgrade your guitar and amp first. Maybe an overdirve would be good.
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This may sound weird, but I throw an acoustic sim. on sometimes. You can really screw with your tone with one of those, and get some really funky sounds.
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