Wow what an extremely hard question to ask.
If I only was able to use 1 pedal it would be my Boss Wazacraft analog or DD-7 Digital delay pedal. I would have a hard time not using both as I love both of them so much and can get exactly what I want out of either of them. I like using 2 or even more delay pedals as I can tone shape and write without restriction.
Man but I would be lost without my Boss Waza Chorus pedal. Yeah call me old school 80's style player but I use Chorus on cleans most of the time. It's pretty rare I turn it off when I'm doing clean work. A few of my songs I have tried without and it sound beautiful and full but when you kick the Chorus on man it gets deep.
If I was to have a 2nd pedal , this would be the one.
Now up front of the amp If I only had one pedal it would absolutely be the tc electronics Spark Booster. This damn thing is so wonderful for anything you would use a boost or soft OD pedal for. Man it's incredible and organic sounding. It really brings you to a play of inspirational emotions. It's rich and lush accurate and very fast.
The wah of course. A cry baby. Not that i have any other pedals. Not that i wouldn't want them - it's just clear to me that the wah is more important since it's a dynamic interactive foot control and not just an on switch. The volume pedal could be a contender, but it doesn't add as much complexity, plus the regular volume knob offers much of the same functionality. I don't know of any other expression pedals. Probably many other parameters would be great to have there.
For most stuff, i could make do with freebie VST's and pro-ass tinkering from years of electronic music production. Actually all one really needs is a MIDI expression pedal... But that doesn't change my immense verdict that the expression pedals are more essential and expressive (duhh) and instrument-ey than the button boxes.
I also feel that the talkbox isn't used nearly enough.
Empress Compressor
Tech21 BDDI.
I literally cannot play without those 2.
I don't use wah a lot, I find it far too overused in as it is. And I hate talkbox sound, can't name a single song where it actually sounds good.
But otherwise I fully agree. Expression pedal option should be standard in effect pedals.
Hve you checked out EHX Worm? (I know I've been raving about it here probably too much already, but it's just so brilliant device... And i'm usually very critical about gear)