The criteria for the same hasn't been stated clearly. The Grand Priest only stated that the Universe with the Most fighters remaining in the end would be the winner and the Best performing fighter would receive the Super Dragon Balls. Generally, the last man standing would most likely be the best fighter. However, this might not be true considering Freiza who is weaker than Goku and Jiren is still standing.
However, it is most likely that the Super Dragon Balls are going to be given to the last man standing because the other scenarios aren't possible.The fighter with the most eliminations get the Super Dragon Balls: This is not possible as if the fighter happens to be from a Universe which is eliminated, he would be erased. Example:(Suppose Jiren wins and Universe 11 wins, Goku and Vegeta would still have the most eliminations but they wouldn't get the super dragon balls as they would be erased.)The strongest fighter: It is fair to assume that the strongest fighter wouldn't be eliminated as there isn't anyone strong enough to eliminate him. The only way he wouldn't obtain the Super Dragon Balls is if the other team has more fighters remaining. Then the Super Dragon Ball would most likely be given to the strongest/ best-performing fighter on the opposing team. Example:(Suppose Jiren is still stronger than Goku and doesn't get eliminated. Also, Goku and Freiza remain in the end. Jiren wouldn't get the Super Dragon Balls even if he happens to be stronger as Universe 7 would have more fighters).The last man standing: This is most likely the criteria as there aren't any special cases like the other 2 scenarios where someone who doesn't clearly fit the criteria isn't rewarded the Super Dragon Balls. This only problem with this case would be if equal number of fighters from more than 1 Universe remain. However, it could easily be solved by having them fight until only 1 fighter is left.