Valentino, Marc Jacobs and Tom Ford were the best in terms of mentoring young talents and making them grow:
From Valentino we got Pierpaolo and Maria Grazia
From Tom Ford we got Clare, Frida, Alessandra, Phoebe, Peter Hawkings
From Marc Jacobs we got Demna
From Karl we can say we had an early Alessandro Michele and Frida as they both worked at Fendi before leaving for Gucci LOL
I would have said Tom Ford and Nicolas.
From Tom, we had: Stefano, Frida, Alessandro, Clare, Stefano, Milan Vukmirovic, Christopher Bailey, Francesco Costa, Francesco Russo, Vanessa Seward, John Ray and yes, Peter Hawkings.
From Nicolas we had: Bouchra Jarrar, Sébastien Peigne, Barnabé Hardy, Natacha Ramsey Levi, Julien Dossena, Nicolas Di Felice, Charlotte Chesnais.
Regarding Karl, I think everybody who worked in Italy worked at Fendi at some point. Pierpaolo and MGC also worked at Fendi. Vincent Darre was Karl’s assistant at Fendi. But I think the best talents who came from Karl were Herve Léger and Victoire de Castellane. But a lot of fashion Parisian girls started with Karl at Chanel.
Marc Jacobs: Demna, ok but Peter Copping , Julie de Libran, Camille Miceli and to some degree Kim Jones.
Roberto Cavalli also was great for young designers. I think about Olivier Rousteing, Peter Dundas or Lorenzo Serafini.
Then you someone like Yvan Mispelaere who went to Valentino, Prada, Gucci (under Frida), Chloe (under Phoebe) and Valentino again under PPP.