The one on the left is in front. You can tell because the sun is to the left which is causing the shadows on the right building to go towards the right.

It seems some details of the building on the right are missing so that points to the one on the left being in front.

Laurel! And the dress is blue!

If you look VERY closely, you can see that the building on the left causes some inconsistencies with the imagery of the building on the right. Come of it overlaps the right's windows, suggesting that the right is actually behind the left. But the lighting can be VERY misleading.

The one on the left is in front of the one on the right. It's only the upper floors that give it away.

I say left because it's a little sharper, the one one the right looks out of focus, suggesting it's behind.

Left

Assuming "the front " is where we are looking at it from, the gray building balconies cover the windows of the brighter building, while the brighter building only appears to cover to cover the gray building in the cut out areas. Or...I'm blind