It depends on where you are. I would probably just wave in the air and say, 'all of them' if asked. If they push back, which they will, I turn it around and ask them: which murders and war crimes were explicitly opposed by the party you want me to vote for? They don't have many options other than to lie, make something up, or brush it off as a kind of 'realpolitik' necessity. If they do have a good answer, I'll ask them about the murders and war crimes that their party didn't oppose and ask for an explanation. Either way they will have revealed their character and reinforced the pointlessness of voting for their candidate.

Edit: I'm not suggesting this is the only way or my only response. It was a bit tongue in cheek. Depending on the person, I'll do a mixture of some of the other things suggested in this thread. For the people willing to listen, I might explain the commodity form and explain why neither of the main parties represent my interests. But mostly the people I talk to IRL already know what my views are and there's either no need to bore them because they agree already or there's no need to bore them because we both already know that we'll never agree.