Highest THC is impossible to say. Every experienced grower will tell you it all depends on the conditions it is grown in, and how well cared for the plant is. If you stress the shit out of a white widow and don't treat it properly, you'll get under 20%, but care for it properly and you'll get 20's.
Really many strains are on par under the right conditions: Nothern lights, cheese strains, white widow, LSD, all of which have very comparible THC levels if cared for properly. Then again, these are all mainstream strains... there's always some Dutch man who has created his own ridiculous strain... it's impossible to say.
If you are talking easiest, autos. No worry of light changes, quick seed-harvest, hardy as fuck and forgiving. Lowryders, Afghan K Ryder etc.
Goodluck mate