I can map montreal by its pastry shops, and I have regular stops like Gascogne ... but must tries:
patisserie G&G on Mont Royal street just east of Papineau. Since she makes everything fresh, the offers of the day may be limited in the store - so do consider ordering something from the extensive catalogue. My coups de coeur - is a vanilla mango combination - it is just heavenly. You can check the website and download the entire catalogue (click on commander). Geraldine is an exceptional patissière.
the new Olivier Potier has been mentioned in other posts. http://olivierpotier.com/
Maison Boulud (at Ritz) is worth it - do not skip the exceptional desserts of Suyin Wong.