That is a close call between three regens:
Francisco Escribano (FLC) a CM0304 regen who entered my Europa de Nava youth setup round 2019. He singlehandedly took us from 2nd Division B to Champions League Glory.
Ricardinho da Souza Alves (ST) a FM05 regen I was introduced to when I took the Auxerre job circa 2023 IIRC. He was already 26 and had already won loads of individual prices. I was unable to guide him and Auxerre to glory, but I still swear by him when trying to find a new striker. I kid you not, he was
el baixinho reborn.
Jamie Taylor (AM) a FM06 regen who entered perennial champions Workington side through their youth setup in 2061 at the tender age of 14. Even at that short age you could already see he was going to be something special. Started to play 1st team football at the highest european stage at the tender age of 17. At 19 he was already full international for England and Workington's playmaker. When he retired in 2081 (after winning all that can be won at club and nationa level) he entered Workington's coaching staff as assistant manager.
I still shed a tear now and then for those players