In general:
** Brown or Polar bear,
** Elephant,
** Cape Buffalo,
** African Lion....

All to be hunted on foot (on the ground) and away from any man made protection (other than your weapons)

AND, here is the kicker....

Chose the one that in your heart of hearts you are most fearful of....

In my case that would be African lion (perhaps because of the video clips I have seen) although I grant the other three have the ability to be plenty troublesome.

JMHO,

EKM