The OG Ghost Train Haze is at a whopping 25.79%, according to the May issue of High Times magazine entitled "The Strongest Strains On Earth" ... but these were the results from the cup, they didn't even list strains that were below 20% thc content, the thing is that THC is not the only contributing factor in potency. 1st of all, it depends on what you're looking for and how you're measuring the potency of your smokes. There are many, many cannabinoids in cannabis that have a psychotropic effect on your brain... there are even strains that harness cannabinoids that aren't found in other strains; also consider the fact that cannabidiol blocks the effects of THC to a degree but works well for medicine because of it's tiring/munchies qualities. You could have a strain that tests 25%+ but if it were to have very high levels of any of the cannabinoids that fight against the effects of THC, then it wouldn't seem as "potent" if the THC content is what's considered potency.
The best strains I have messed with to date:
Serious Seeds: AK47, just about anything
DNA Genetics: Lemon Skunk, R.K.S.
Reserva Privada: The OG #18
Emerald Triangle: Grapefruit Krush, Trinity Kush and Emerald Jack (Emerald OG Kush X Jack Herer)
There are waay too many variables (strain, environment, growing methods, tolerancy, time flowered, time cured, method of ingestion, etc.) for anyone to pick a definite "most potent strain", especially since good growers are few and far between.