* A lemon: This is the most straightforward and common idiom.
* A dud: This emphasizes the lack of value or quality.
* A clunker: This suggests something old, worn out, and unreliable.
* A piece of junk: This is a harsher term, suggesting something worthless.
* A white elephant: This refers to something expensive and useless, but it's not as directly related to quality as the other options.
The best idiom to use depends on the context and the specific nuance you want to convey.