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Hey mate it's not really a question of highest 'quality' of cannabinoids but highest percentage or ratio.
To put it simply rec users want high percentage THC strains whilst for medical patients they will be looking for in most cases low THC high CBD strains. I use one which is equal parts CBD/THC, contains 7.5% of both. I find it good for pain relief, insomnia and anti anxiety. Some patients will prefer no THC and stronger CBD strains, depending on condition.
All the cannabinoids and terpenes work together. We still know very little about the plant.

They are getting up to about 12% CBD strains as far as I know. Breeders in the States are hitting 30% THC strains. Very strong.
OG Kush is around 24% THC and low CBD off the top of my head. A strong strain which the rec users like.
The reason OG Kush is so popular is also to do with branding. It has a fairly famous lineage of strains it's descended from and also the obligatory myths of who and where it was created. I think it comes from the Emerald triangle in NorCal from memory.
I'm very interested to find out what strains UG come out with.
One snippet of info is I can tell you that the head Gardner at UG used to work for Canopy.