My vote goes for the Surface Book, I got a Surface Pro issued at work while a colleague got the Book, the Book is way better.
It fits into your requirements of 15", Dedicated GPU, 16GB ram however it doesn't take more than 1TB as standard AFAIK but it's got USB-C and 3 so external shouldn't be an issue. The issue is price, it can hit $3,500 depending on how spec'd you want it, as a corporate laptop hell yeah, on the personal side, a bit expensive.
It fits into your requirements of 15", Dedicated GPU, 16GB ram however it doesn't take more than 1TB as standard AFAIK but it's got USB-C and 3 so external shouldn't be an issue. The issue is price, it can hit $3,500 depending on how spec'd you want it, as a corporate laptop hell yeah, on the personal side, a bit expensive.