If Pant regains his usual form and fitness then he would be the strongest contender for captaincy but only in Red ball cricket because for red ball you have to consider Home/Away conditions as well and he fits the bill for every condition. Rahul can challenge him in the test format if he starts working on his wicket-keeping, especially on turning wickets, plus agrees to bat in the middle order.
Yeah. I feel that he would be a strong contender not just in tests, but in the T-20 format too. I see Pant, Rahul and Iyer being great potential contenders for captaincy in the future once Rohit steps down.
Hardik was pretty average as a captain so far which is why I feel that he should just focus on playing as an all-rounder which is what he does best.