Way 1 · best default
I'm not sure that's a good idea
Honest and clear, but "not sure" stops it sounding like a judgement.
Examples
I'm not sure that's a good idea. The kitchen's already quite dark.
Painting it black? I'm not sure that's a good idea.
In a conversation
A: I'm going to quit tomorrow and tell my boss exactly what I think.
B: I'm not sure that's a good idea. Why not sleep on it first?
Common mistake
Not: Your idea is bad.
Say: I'm not sure that's a good idea.
Calling the idea bad sounds like you're calling the person bad. Doubt is kinder and just as clear.