I'm assuming we aren't supposed to mention multis, but if that's not the case, I'm getting more use out of my BP355 than I ever thought I would in live settings. I've also done a lot of reworking its patch bank into a series of fun effect combos, useful EQ switches, noise FX, etc, so it's basically a custom pedalboard all in one rather than the hundred massively disparate, often illogically configured patch setups.
But if no multi...then hrm. The only other pedal I've used live so far is my preamp. But I have a Q-Tron Plus that I'm just ITCHING to have an excuse to bust out, and am pretty sure that once I nab a compressor (looking at the Empress), that's going to be on all the time.