Murray's forehand for sure. His backhand was always his beast shot and imo one of the best backhands in the game currently but he has a brutal forehand now too.
Djokovic's mental focus. The way he turns matches around When he's down is unreal these days. I admit i didnt watch him early on in his career but the way he just switches it on is amazing to watch now.
To be a tennis pro, you have to get good at handling players balls.