I have many pairs of the UGG "Classic" boots, both in the short and tall height.
These boots are absolutely "the best" for warmth and I live In Canada, so I know about cold and snow!
HOWEVER, Please order ONE SIZE SMALLER than you usually  wear because this style runs LARGE!
I wear an 8.5B or a 9B in other boots and shoes but the 8B is a PERFECT fit for for me in this style of UGG boot.
CAUTION: This advice ONLY pertains to this particular style of UGG boot. Some other styles, like the Bailey, run smaller, for those, you need to order one size larger!
Hope thas helpful and not too confusing!