A plant cant produce past its genetics. That is why having good genetics is more important than having better grow skills. You can be the best grower and grow bad genetics but it won't be as potent as the best genetics on a mediocre grower.
here is a list of some rare hyper-potent strains only available through clones:
Cali-o, Matanuska Valley Thunder F***, Champaign, Chemo, and G13.
I had a friend who got his hands on some purple Champaign and he said it was the best weed he ever smoked. That wasn't even the pure form too. I am sure the other strains such as White Rhino and Ice have a lot too. The most potent one that is easy to grow has to be Ak-47 though.