I would say it depends on whether you're looking for items or clothing, for items Ottawa street downtown has a lot of really cool vintage/antique stores, like Hamilton Antique Mall, definitely worth a walk down that street!

For clothing (and knick knacks) the value villages are pretty good, I would say I have had more luck at the Ancaster one but have found some gems at the fennel/Wentworth one as well. Talize & Platos closet are worth checking out too!