For me its between the 1st and the 3rd.
The 1st because... well they were the first, the originals and probably the most powerful leaders their villages ever had (judging from Hashirama. the fact that they established a village and order around them and the 1st Kazekage gattering desert-dwelling shinobi through his strenght).
The 3rd because three of them were said to be the strongest ever (with Hiruzen being retconned in part 2 but still...) and most of them are above their 40's, showing that they're strong enough to live that long.