So the XPS is the classic best build but does it really pay off? Let's get the following NUGGET in here:

'For thin and light laptops Nvidia offer a Max-Q called version of the GTX 1060 with lower TDP and also significantly lower performance. Sadly this variant im sold with the same GTX 1060 name, making it hard to judge the performance.'

So this means the WINNER here is the Alienware 15 R4 since most of what you noted pushes us to the best GPU in the lot. RAM is a non-issue as we can fix that where the GPU can't be changed later.

Winner? Alienware 15 R4