Here's a quick list I made of all the strains I could find with claimed THC contents over 20%. Strain name first, then seed company, then a bit of info:
Sex (THC Laboratories)
27.3% THC, 1.0% CBD, sativa
Hawaiian Snow (Green House Seeds)
23.7% THC, 1.1% CBD, 0.8% CBN, sativa
Arjan's Haze #1...