I would definitely say they are the most prevalent in the cocaine trade, because their cocaine trafficking activities reach into a wide range of countries. They are literally everywhere. Yet they have little power in a couple of large markets, such as UK and Ireland. They have more activity in Netherlands and France, but also in these countries they don't necessarily run the show.
There is no criminal group that I would say is 'the most powerful', since there are a lot of groups active in differents areas.