If we're talking about any active "groups" in Belgium? As far as I've come across:

Italians are #1
Belgium has a large Italian population of Sicilian, Calabrian and Neapolitan background (as well as a smaller amount of people from the Marche and Abruzze regions as well as Sardinia). Families and criminals linked to more than a handful of Sicilian mafia, Ndrangheta and Camorra clans are all active in Belgium and are all locally heavily involved in a wide range of criminal activities.

Dutch groups settle on a (maybe to some of you surprising) second place. Most will tell you that almost any Dutch criminal firm active over here has roots in the trailer parks of North Brabant and South Holland, but this isn't necessarily always the case. "Reizigers" (which is how they call indigenous Dutch people with roots in the trailer parks) are definitely involved, but so are many settled Dutch. Basically any border area in Flanders has a Dutch criminal firm active. They mostly concentrate on the drug trade (though some will dabble in other criminal activities as well) and, like the Italians, are pretty smart about it.

Groups from a Moroccan origin are mostly active in the big cities with a large Moroccan population. Antwerp, Brussels, the Charleroi region... On an organized scale, Antwerp is probably their main ground zero because more than a few families and criminals over there have moved beyond the usual "Moroccan petty drug dealer" stereotype and have been active in the reasonably higher level of narcotics import for a long time now. In general their role has become a bit exaggerated by the media though.

"Russian" groups made a lot of noise in the 90's, but they've been keeping quiet for a long time now. Back in the 90's they were mostly active in Antwerp and basically none of them were actually "Russian": the majority being Georgians, with a few groups of Chechen (who were mostly renowned for violent extortion), Armenian and Jewish background. Back in the 90's these groups were involved in a fair share of vices: a LOT of trade in counterfeit goods, extortion, prostitution, some fraud and robbery schemes, the occasional drug trade... Don't think they're highly active nowadays. Back in the 90's these groups had actual visible "turf", and those areas were mostly rather vibrant, but nowadays those same areas are completely dull and lifeless.

Turkish groups are active in basically any reasonably sized Turkish community. Most are divided due to their region of origin. For instance in North London and The Hague the Turkish OC is mostly controlled by criminals of a Kurdish background. Over here there aren't really a ton of Kurdish Turks, most of the Turks living here being from an ethnic Turkish (a ton of them being from the Black Sea region as well as the more central parts) background. Active in a variety of stuff.

Albanian groups made a lot of noise in the 90's and the early 2000's, but the past few years they've cooled down considerably. They're still active of course, mostly in Antwerp as well as Brussels and parts of Wallonia. They're involved a lot in prostitution and extortion (though not as frequent as in the 90's) as well as the occasional other vices.

Chinese OC is active in Antwerp's Chinatown. When I took a look around over there I was actually surprised how many "dodgy" types were just freely walking around over there.

Maybe another surprise to some, but many Greek communities over here have their own groups active as well. They're not loud and showy, but they're definitely active.

Occasionally a high level Irish or British gangster pops up in mostly Antwerp (there's at least one case of a powerful Dublin crime boss owning a pub over there), but they don't really conduct any blue-collar criminal activities here.

Outlaw motorcycle groups are active. The OMCG crews involved in more organized criminal activity here are mostly regulated from the Netherlands.