Yep, I'll also agree with st0ner and ArtRollins. It's pretty much impossible to say that any one particluar strain is the strongest because a lot depends on how its grown and can differ from plant to plant. Even if you take a strain like white rhino you'll get crap if you just thow the seeds on the ground in a field somewhere. You can't expect the same results as someone who grows them indoors with months of TLC.

Any of the strains commonly offered at the cannabis clubs for medical use are all super potent. Most have THC levels of 20% or more . Some of the ones I've tried from a med mj supplier are

Blueberry
White Widow
Sour Deisel
Jack Herrer
Juicy Fruit
Green Dragon
Manatuska Citrus

All of these will get you pretty stoned with one hit. Two or three hits and your toast.