Everyone says that the best "frites" to be found in Brussels are at Maison Antoine on the Place Jourdan. I don't agree... they're overrated and you always have to queue for ages. Instead try the much less clichéd and much more tasty Frites Flagey at Place Flagey, just near to the hip Café Belga and the venue Flagey "factory of sound and images", a cinema and concert hall. Not only are the chips better, there is almost never a queue, there is service with a genuine smile (a rarity at Maison Antoine) and they are cheaper too!
Frites Flagey, Place Flagey
Tram 81, 82, stop Flagey
Updates
Though we never tried the chips the place itself didn't bode well. It's little more than a Portakabin on the edge of Place Flagey, which looks like a bomb hit as it's in the middle of major works. Not a pretty place at all. This might explain why there is never a queue.
Is this your business?