The behemoth I commissioned was mainly distortions drives and boosts, Just a hugely expensive noise box really.
For me the ultimate multi effect would have at least 8 switches, assignable in any way I liked, parallel processing, a good compressor, some great synth sounds, a clean boost separate from distortion and fuzz, a really bubbly watery auto wah, a pedal for manual wah, a phaser, a chorus and a flanger. It would be instantly responsive (no lag) and have external control via a smart phone or tablet. Be bluetooth compatible and with a proper DI out. Be as physically small as possible, rugged construction and a really simple user interface.