I used to use arrow keys, but it felt too crammed up. Now I use the traditional WASD keys
Shift is reload for me, because I am adapted to the idea of the shift button being reload. I can't get used to 'R', and it prevents me from having to move any finger off their designated button to reload.. My pinky finger is idle until I reload
F is drop, mainly because I rarely ever choose to drop a weapon.
space is prone because it gives something for my idle thumb during gameplay.
I try to stick to the idea of not overloading functions on a finger. Keep it simpler for my hands.