What is best as Greek yogurt substitute?

i've subbed Greek yogurt thinned with water (not milk, though why not) and its worked great.

Agreed, I would try Greek yogurt thinned with milk! I think that will be the closest to regular yogurt. I haven't tried subbing sour cream in this particular cake, but I think the recipe is forgiving enough that it could work just as well.

I would try the latter, Greek yogurt thinned with a little milk, since Greek yogurt is simply yogurt that has been strained for a period of time to thicken it by removing excess liquid. I would err on the side of less-liquid-than-you-think and then see whether the consistency of the batter appears normal; in any case I really don't think you can go wrong here.