Where to start!
All the Trappists are solid enough and then there's Dupont and Duvel who are pretty well available. Kwak and Tripel Karemliet from Bosteels along with Delerium are worth checking out.
Most of the Abbey stuff, Grimbergen, Leffe, St Feuillen, Kasteel etc. is Ok, but nothing too special, with the exception of St Bernardus, especially their Abt 12.
Fro something a bit out of the ordinary there are three breweries which make really sensational stuff: Cantillon in Brussels, Fantome in the Ardennes and De Struise in West Flanders.
Anything from these three is worth a try, although can be a bit difficult to track down.
Cantillon in particular commands ridiculous prices on the secondary market. Check this out. 3500 dollars for two bottles of beer...
http://mybeercellar.com/?itm=54228