I've been getting medical marijuana from a single legitimate (legal) source here in San Francisco for a couple of years. It's all nicely grown and cured, no seeds, organic, etc. From what I remember of the different strains, rating from most to least potent :

Mind boggling "wtf did I just smoke?" strains
Purple Erkle
Manatuska Thunderfuck

"Damn, this is some really potent weed" strains
Green Erkle
Sour Diesel
Jack Herer (probably the most potent predominantly sativa strain I've tried)
White Widow
Black Domina
Afghan

Still "Dank" but nothing special strains
Blueberry
Juicy Fruit
Pot o' Gold
Green Dragon (yes, an actual strain named greed dragon)

At least that's my experience

Oh yeah, like Stinky said, Sensimilla just means "without seeds". That may the slang term for seriously dank weed in some place but it's not a specific strain.