here's a couple blurbs about thc levels from reddit:
Double Grape at 28%....White Crack at 27%...Forgotten Cookies & Skyrone Stomper at 26%...Gold Glue,Cosmic Queen,and Samsquanch at 25%
Forgotten cookies - 26.3% Samsquanch - 25.3% Cosmic queen - 25.3% Double grape 22.6% (but if I remember correctly there’s a 27% somewhere) Ripleys og - 23.1%
There’s been a mid 20’s Chemdogging and AVT also I believe.
It's possible that the best cola from the best bud could test that high, but on average those strains are in the same 15-20% category as most potent strains.
I've grown DoubleGrape and WhiteCrack, and they weren't above 20% thc.
I grow mostly indicas, and Skywalker is the strongest mephisto indica I've found so far.