I don't think you can talk about one criminal organization being the most powerful. A couple of organized crime groups each have their own interests in different locations. As for being the most prevalent? I would definitely say the Ndrangheta. In Belgium Ndrangheta groups have been able to blend into local Italian communities, operating from small local bars, restaurants and pizzeria's. Their main activity is cocaine importation (smaller amounts of marijuana and heroin as well). They import the stuff but within smaller communities they work as drug pushers themselves as well. Extortion has also been noticed. Ndrangheta guys aren't typically flashy, they mostly come across as gritty born and raised hoods.
Other groups in Belgium include Turkish mobs (work mostly within their communities), Moroccan crime families, Albanian mafia, Greek crime groups (literally every Greek community has at least one) and ex-Soviet crime groups (largely Armenians, Russian Jews, Georgians and Georgian Jews). Native Belgian groups are mostly active in Liège and are mostly linked to Italian groups (it used to be the Sicilian mafia, but not too sure what the state is on them now).