The problem I have with that is shit talking your previous work is generally seen as unprofessional - yet, you're also leaving them for a reason.

I would probably be evasive if asked your question (at least, in terms of 'least favourite'), which also wouldn't work in my favour.

Why does it have to be shit talking?

Surely there are value neutral things you didnt love about your old company.

We couldn't install our own tools which hampered creative problem solving.

We weren't given enough direct access to our customers to make better design decisions in the product.

We didn't have a strong culture of documentation so we relied too much on tribal knowledge.

You would have to take the answer in context, ie does it align with their reason for leaving. Shit talking would be a red flag but everyone's got unique cultural preferences. Someone's favorite part might be someone's least favorite part from that same company.

I never ask that question about the candidate's current or most recent previous employer. Any answers will be guarded or clouded.

Instead I ask what coworkers they still remember from their earlier jobs, and why. This has the double benefit of putting enough emotional distance between now the time they will be talking about, and it highlights the traits the candidate considers important in others. It doesn't even matter if they point out the negative memories over positive ones.

There is one intensely high-signal question but I can only ask that with other Finns. Namely: "What university courses did you cheat on?"