If I remember right, Anderson said it was Celtic in the special features. It kinda shows in Celtic's cocky personality in the movie. And how he's always infront of the group. Celtic seemed to know what he was doing when he planned out the attack on everyone from inside the pyrimid. He showed leadership.
I think if Celtic survived he would be even more badass than he already is. He would've been all cocky and survived, of coure. It's kinda stupid to create such a very likible character such as Celtic and kill him off like they did. Just to point out, being fearless and jumping into the danger is what earns Predator their rights, honor, and respect apparently.
Scar wasn't as cocky or as great as a leader Celtic was but he was very independent. How he hunted with his spear by himself. And of course making his way out of the pyrimid as the only surviving Predator, not to mention he was also the only Predator to retrieve the Plasma Castor.
Chopper had the potential to be bad ass but Anderson never excuted it. It was a big shame. Not only that but it was the scene where the Predator and Alien, two legendary creatures, meet for the first time and its only seconds from the first Predator's death. We never really learned much from Chopper or his character but he seemed to be alot like Scar in the fact he stayed behind Celtic and did his own thing, well atleast tried.
I think Celtic is the leader is any.