It depends on what you mean by "powerful"? If you are simply talking about THC content, since testing and growing conditions vary, you will probably only get three or four pages of arguments over twenty different strains of which is better and over-exagerated claims of why they are better. If you are looking for "powerful", as in pain relief, I'd start my search in the MMJ Strain Reviews section where you will find info on 100's of different strains that will help. The old stand by, Northern Lights, for me, is the most "powerful" sleep aid I have found.
look at the cover of high times magazine for your answer(s) the article earths strongest strains list a few over 20%
here is one:
OG GHOST TRAIN HAZE
OG KUSH X (TRAINWRECK X NEVILLE'S HAZE)
highest % of THC at 2011 high times cup event Denver CO
Rare Dankness Genetics