The "best" will be Joystick Gremlin, because have all features of the four.

The use of one of the four depends on game and player needs, because each have their own limitations:

I use JoyToKey for "mouse emulation" in joystick POV HAT for overcome the obsolete camera control of IL-2 Cliffs of Dover, can use XPader too, but their interface is a bit cumbersome for teh same result.

But I can't use JoyToKey for IL-2 Great Battles (Bo'X') because the way this game see keypress result in problems with combos, e.g. LCtrl+A, the game see LCtrl+A and A.

For this game RSMapper of SVMapper work OK, but both don't allow map keypress in axes, what JoyToKey and XPadder do.

XPadder and RSMapper allow set an delay between key press, allowing create sequences for commands in just one button, e.g. open de canopy and the bail out (available in some flight games), JoyToKey and SVMapper no.