we cant really answer that. depends on breeder, grower, and handling of the finished product. and also, with all the strains they are coming out with, its really hard to say. some say its still White Widow, but that may have changed. my guess would be some of the white strains, some of the hybrids like jack herer. chemdog i think is up there. some say sour d. its such a crap shoot, thats probably why i am the first person to post it. also, you may want to move it to a different section...
even with a potent strain the variables include the gardener.. so this question is tough to answer...
You can get a general idea of potency from sites like the single seed centre where you buy seeds. I'm not saying that all that info will be credible all the time, but as a general guide it's as good as anything else. Like the guys above said, there are tons of factors throughout the grow which can affect potency, but if you're looking at it as if all things being equal, which are most potent, then you might check out that site.