I do not wish to get into a fight with anyone but may I suggest that those who find this tread annoying and/or pointless just ignore it, instead of posting nasty comments.
Maria, you who started this tread, I will say that beauty implies not only physical appearance but also charisma, personality, values and intelligence (both emotional and intellectual).
On that basis, I would say Tatiana was the most beautiful. In addition of her fine features and natural dignity, she was incredibly selfless, dedicated and generous. A beautiful soul.
But on the overall, Nicolas and Alix were blessed with four gracious daughters, all in their own way. So the choice is a hard one to make.