I buy most of the parts I need to build computers for myself and my friends from Infonec's Markham location which is just north of Steeles and West of Warden.
They do not have as many customers as, say Canada Computers, so the service is more personal. I bought a midrange MSI Video Card once from Infonec that started having problems after a few months of use. I returned it to Infonec and since they know me, or at least since all the purchases I have made are recorded on their computers, they took it upon themselves to send MSI the video card and get a replacement. They then called me when the replacement arrived so I can pick it up.
I also order stuff from Infonec that is hard to find elsewhere in Toronto. For example, I asked them to source out a Zalman GT1000 PC case and they got it to me after a few days at a price cheaper than if I were to order it online at NCIX.
I also highly recommended Infonec.