There are quite a few fast finishing strains but 6 weeks is really pushing it,every really good strain ive ever grown ,even if its supposed to be a fast finisher dont finish that fast,not if you want primo bud.
I could reccomend a few i know of but i have a feeling from the 'well known" statement that they would not be the impressive strains your looking for,not trying to be a dick just honest,most of the strains your thinking about will not finish in the time you want,8 to 10 weeks is what it takes me to finish & several times its taken 10 to 12 weeks.
If your serious take this link & research all the strains from different breeders,most available strains are covered in this link.
http://www.drchronic.com/