I have two...a quality ABY that can flip phase, lift ground and isolate signals...and a reliable, smooth tapered and "scratch-noise" free volume pedal for swells and/or signal muting. My Radial BigShot ABY and my Morley Pro Volume have been really useful workhorses for me over the years.
Don't know if everyone should own it, but this guy has made a night > day difference in my rig, for eliminating hum/buzz and end of chain buffering, in stereo! So under the radar it sits mounted on the underside of my board!
ZVex Super Hard On. I made my own clone. This is a super easy project to build. Sounds great, acts as buffer, or a boost. I use it all the time. Mostly at the end of my signal chain as a get louder button.
If I had to sell my pedals off, it'd be the last to go.
If I had to reorganize, it'd be the first in the new board.
If I had to choose the best pedal I've ever plugged into... it'd likely be the one I choose.
It goes with Fender, Marshall and D-Style amps... so it's likely the most versatile pedal I've ever owned.
I don't really have a favorite. Boss lacks glamour but gets the job done. The MXR 10 band gets some representation in a few places, and you'll see the occasional board with other EQ styled pedals like the Empress ParaEq, EQD Tonejob, Bjorn's Sea Blue EQ, and some people even use their Timmy for this type of thing. A pedal that you use as dedicated EQ has a lot of power but its a lot less exciting than a Fuzz or a Klone.