In "Rikki-Tikki-Tavi," which snake is the biggest challenge to Rikki-tikki?
Rikki-tikki fights three snakes; Karait, a small snake, and Nag and Nagina, the two cobras. Although Karait is described as more dangerous because of his small size and agility, the two cobras are much more dangerous because of their great strength. Rikki-tikki cannot kill them in a single bite, but instead must continue attacking until they wear down:
...[Rikki-tikki] was battered to and fro as a rat is shaken by a dog -- to and fro on the floor, up and down, and round in great circles... He was dizzy, aching, and felt shaken to pieces when something went off like a thunderclap just behind him; a hot wind knocked him senseless, and red fire singed his fur. The big man had been wakened by the noise, and had fired both barrels of a shot-gun into Nag...
(Kipling, "Rikki-Tikki-Tavi," cs.cmu.edu)
Rikki-tikki kills Karait with a single bite, and is assisted in his fight with Nag. Of the three snakes, however, Nagina is the most challenging fight, as Rikki-tikki must follow her into her hole where it is very cramped and there is no room for him to use his speed against her. The fight is not described, but all the other animals assume that he must be killed, and the fight takes a great deal of time. Because of the difficult location of the fight, and that Rikki-tikki must fight her alone without any help, Nagina is his greatest challenge.