That is of course totally dependent on what you are looking for. However, QuÃ©bec City is totally horrid for shopping. I am certain that fatherseanfan, who also lives there, would agree with me totally.
If you want suits, there are two or three options, though they are all overpriced: Holt Renfrew (Zegna, Canali... Hugo Boss too), Simons (Paul Smith, Costume National). Both of these places are located in Place Ste-Foy, a small just across UniversitÃ© Laval, on Boulevard Laurier. There's another Simons location in Old QuÃ©bec but its not worth a damn.
There is also a store named Laflamme or something like that on St-Jean which also carries Zegna, Boss and Versace. Seriously, if shopping's better in MontrÃ©al (Holt Renfrew, which is much bigger in Montreal, Ogilvy, L'Uomo, etc...)
If you are looking for casual clothing, my answers would be the same. You'd be better off shopping elsewhere, unfortunatly.