I'll ignore the really early ones I had that were massive...
I'd say my Sony Ericsson T86i. I'd just come from a Nokia 8210, which is one of my favourite phones ever, and the T68i just seemed to do everything worse. Bigger, heavier, worse battery life, really bad joystick, useless 256 colour screen and worse reception. The T68i isn't the worst phone ever made by a long way, but it's the worst one I've owned.