My most disappointing TF purchase would have to be Universe Silverbolt - especially when I paid full RRP ($60) back when it was released (or close to it).

For those unfamiliar with the mold, it's basically a robot tucked underneath a shell of an airplane fitted with annoying electronic sounds and lights.

No complex transformation whatsoever either, simply fold front half of jet shell upwards, extend legs, push legs up into torso to reveal head and you're done.

I've had Legends class toys more satisfying than this guy.