I don't think Natasha is Miuccia's favorite.
The designers who seem to favor her a lot are :
Gucci (Frida) - 9 Gucci campaigns
D&G - She has continually opened and closed their shows and is very close with them.
Givenchy (Tisci) - They are also very close.
Stella McCartney - I think Stella loves her and has spoken highly about her.
Alexander Wang
Jil Sander (Raf Simmons) - 3 campaigns in a row. Also, Raf has mentioned how Natasha is the epitome of his aesthetic or something along those lines.
Blumarine - Opened and closed the shows multiple times plus 2 campaigns