My vote would have to go to the Trendmasters 6" Mecha-King Ghidorah toy. I got it as a kid because it looked cool, but soon found that it was absolutely useless as both a plaything and as a display piece. The middle head and wings were a separate piece of plastic from the rest of the body that was held on with a very small, flimsy tab that didn't serve its function in the slightest. The moment the toy was out of the box it fell into the two pieces and would never stay together on its own again. Shortly after that both wings fell off and the two missile-launchers on the sides of the body (which always looks like pathetic, comical little arms to me) became so loose on their joints that they couldn't stay in any position other that facing straight down. After that, the firing mechanisms in both stopped working, as did the one on the figure's back. Around the same time, all of the little chrome pieces that clip onto the figure's necks and legs lost all of their paint and lost their grip, meaning they wouldn't stay in place and fell off constantly. Also the tail refused to stay in its place and fell out continuously as well.
So now most of the time I have a toy that consists of King Ghidorah's torso, legs, and two of his heads, next to a pile of assorted pieces that, with some superglue and fury, might resemble a monster again. Trendmasters toys aren't exactly known for their high quality, but this one really takes the cake. I've never seen a toy fall apart so thoroughly from so little playtime.