Without a doubt the most famous ever winners of an U17 European Championships title are Ilias Iliadis (GRE) and Elnur Mammadli (AZE). Both judokas won the Olympic title. Iliadis in 2004 after winning the U17 EChs in 2002, Mammadli last year in Beijing, 4 years later than his European cadet title.
Worth mentioning other famous fighters who were successful continuing their career are Anett Meszaros (HUN winner 2003), Nijat Shikhalizade (AZE winner in 2004). They were both able to already win a World Championships medal.