Everyone is going to have a different opinion and in the case of Romero and Gabriel I have only watched them for 5-6 games. If I was to rate the back line in order I would go the below.
1 Romero
2 Gabriel
3 Dier
4 White
6 Tierney/Regulion (both have weaknesses)
7 Davies
8 Emerson/Tomiyasu
Not much difference to me. Big diff is the managers, Conte has had only a short time but I expect him to get more out of his defenders than Arteta.
No way is Dier above White in my view. I also think putting Davies above Tomiyasu is a little homerish as well.