Hi there,
In my opinion the best value prepaid sim on the Belgian market is a MVNO on Base. You can order a sim online at:
www.simyo.be
The rates are extremely good compared to most other prepaids which cost as much as EUR 0.40 /min (charged by the minute!)
I did hear they sometimes create problems sending the sim abroad but I've done it several times so if the automatic order system gives any hassle then drop them an email.