Best overall player Federer is a given. No apparent weakness.
You could also make a case for best one-handed backhand in his prime but there are other players who seem to have equal or better OHB's (Wawrinka, Almagro, Gasquet).
Lendl had some specialties I would single out. He certainly had the best OHB during his time but this standout shot was the running passing shot with the forehand which naturally came into heavy use with so many top S&V players around. And mentally he was also at the top, much like Nole is today.
Just an example against Delpo, having 3 set points at 5-6 and not being able to convert them but playing a strong tie break right afterwards. That's mental
