Thick, Greek yogurt is best here. It means you'll end up with a dough that's easy to work with and not sticky. I find that whenever I use plain yogurt that's a lot runnier, the dough takes a while to come together and I end up having to add a lot more flour. So stick to Greek if you can. If you can only find plain yogurt, you can drain some of the liquid away using a fine sieve lined with cheesecloth. Let it sit in the sieve for about 10 minutes then use it.