IMO, the very finest powder available here in the UK [[or anywhere else] is the Swiss stuf. It is nearest the original high-quality black powder made in the latter-half of the 19th century, and much superior to Kranks. It costs more, yes, but uses around 10% less to achieve the same velocity in an arm in good condition. It is THE powder of choice of ALL international competitors on both sides of the pond.
Over in N. America, we tend to use the most readily obtainable, and buy it in lots to save shipping costs, so Go-Ex is the most popular for the millions of BP shooters there, although the serious among them use Swiss, especially the bench and chunk gun shooters.
Much has been opined about the quality of BP since the demise of Nobel and Curtis & Harvey, mainly to do with the apparently varying quality of the stuff from the top of the can to the bottom, necessitating the use of a fine seive to try and maintain the grain size of whatever it is meant to be.
You takes your pick, and spends your money.
tac