Should one be honest about the real reason for leaving previous jobs when asked?

If give a politically-correct standard template answer, interviewers/HR would know it is BS.
If give the real reason, it will be seen as talking bad about ex-employer.

Either way seems unacceptable.

But honestly, if a company is good, no one would quit. Obviously something is bad enough to drives people away and to quit.

How honest should one be?