I'm Belgian. Avoid Leffe, this is a quite bland "Belgian beer". The classic Trappist beers are always a good choice (Orval, Chimay, Rochefort, etc.), but don't sleep on the new microbreweries, their beers can be quite innovative and refreshing
As a Czech I would also suggest to go for smaller brewery, because of our tax system the beer is just negligibly more expensive than from big brewerys.
I will be in Belgium hopefully in about 3 months and because I will bike there I will have a lot of time for tasting. Also I will like to visit some beer festival, if the timing will be right.
Beck's is definitely overrated. I don't know how it's become so popular.
On the other hand, Labskaus is damn delicious, I need to make a vegan variant one day. Labskaus and kale with potatoes and meat variety is what I really miss since turning vegan.