Every time we are in Ghent we go to de Dulle Griet (Vrijdagmarkt 50). It is just a few minutes walk from the historic center. There's a huge selection of beer in a characteristic old pub setting. It is known for a basket and pulley system that is used if you drink one of their beers from one of their large fancy glasses. Due to the high value of the glasses (at least in their opinion), You have to give up one of your shoes and they place it in the basket, pulling up to the ceiling, and ring a bell.
The other bar we tended to go a lot to was Het Waterhuis aan de Bierkant, which is right on the river across from the Marriott. Service is slow but the setting is great and the huge beer list is also awesome. Cheers.