here goes mine:
djokovic’s forehand (watch the 07 us open final and its not hard to notice how massive of a change it was)
wawrinka’s forehand (no need to comment here)
nadal’s forehand
federer’s backhand '17-'18 (that one is questionable, maybe its more due to the larger racquet frame)
Djokovic’s serve, not the one in his 30s, he would still be the best player with his previous serve.
I am talking about the one before 2011. He had arguably the worst serve in the top 20, or even top 50. His first serve was inconsistent, and his second serve was probably worse than Zverev’s.