I don't use many pedals; it's either no pedals, an EQ pedal or a full-on MFX in 4CM.
Which means my must have pedals are:
None.
Source Audio EQ2.
Boss GX-100 (which recently replaced my Line 6 HX-Effects, which I still have and plan to keep).
I love chorus on cleans.
I hate reverb. Always.
edit: I got my GX-100 on sale, but just looked and there's now a $100 price drop, it's $499.99. While it's a digital modeler, I use it for effects only with my tube amps with a loop. Integrated expression pedal, and touchscreen makes it even easier. Has almost every Boss effect, and more; and they sound good IMO. If I purchased all the Boss pedals that I use in my GX100, it would cost around $1200, without the board, case, power supply, cables,...