It depends on the person, really.

For someone who's smart but interested in an actual discussion, I'll usually try to ask them a lot of questions. Like, if they think Bitcoin uses too much energy, I'll ask them if they ever considered that the average kWh in bitcoin mining is greener than the average kWh in other uses, and I'll ask them who gets to determine what a good use of energy is. If they think bitcoin is too slow and not scalable, I'll ask if they've considered that Bitcoin will be used as an underlying settlements layer.

If it's someone who's really stubborn, then I won't bother engaging in a discussion. I'll just re-iterate the main things I like about bitcoin, say something like "Well, I know you have your issues and criticisms, but I believe this has the potential to revolutionize money." and leave it at that.