I like this one, seems to compliment both the backsplash and counter

Perhaps you'd get a few more responses with some more pictures from father back showing more of the feel of the kitchen and cabinets. I'm not a pro but I would be concerned that a grout that doesn't match the tiles would chop up the nice undulating pattern that they present.

I like the grey & pewter the best! Someone did an excellent job on that tiling- very thin grout lines. Not sure if you can tell from the pic but we have a light backsplash and dark grey countertop (not as dark as yours). We went with a light / medium grey and we love how it turned out. Best of luck!

I agree with Skippack, you want the tiles to be the design feature, not the grout lines!

IMO, I would do white no gray/black or other

I agree with those who are suggesting white grout. Either white or a very light gray would be best and won't disrupt the flow of the wavy pattern.

If the tile was not wavy for what ever reason in my head I might go with the very lightest gray (which you can get away with) yet I could more easily say go with lighter gray if the tiles where just plain no movement in them white subway. The wavy aspect makes me think of leaning modern and given that it is why I choose white.

Thanks for all of the input, I really appreciate it. We went with the white grout and it looks fantastic!