I think there are two possibilities for most challenging question a candidate could be asked in their viva.
First, the opening question of the viva. Not knowing what that opener is until it’s asked could make it very challenging. You’ll probably respond to it well, but the anticipation might make it feel tough.
The other possibility for most challenging is whatever question you really don’t want to be asked. Whatever it is, whatever part of your thesis or research, if there’s something you really don’t want to talk about there’s likely to be significant challenges in your mind when it comes to responding.
To help prepare for the first question: remember that your examiners want your viva to go well. They want to help with that by helping you to start well. The first question is likely to be simple stated and reflective – something to get you talking about your work.
To help prepare for the question you don’t want: ask others to ask you it. Prepare. Make notes. Talk about it. Talk about why you don’t want it and invest time in talking about the thing that you don’t want. Hoping you won’t be asked is not enough. Invest time in getting better at talking about it.
You will be asked a first question; you might not be asked about the topic you really don’t want to talk about. Either way, a little prep for both will help you face the challenges of your viva.