I believe C-99 (Cindy99) and herijuana have the most thc with 27-29%. Unfortunately, these are no longer available (I think herijuana is still available, but only in the form of clones...C-99 is only available as a hybrid as far as I know).

A strain with high THC and availability is Hog (I'm pretty sure it got its name from the relaxing effects that the drug that influenced the name "herijuana" is known for).