Don't know if it's the best, but I recommend that you try both the spinach and cheese quiche and the onion and sun-dried tomatoes quiche from L'Escurier (Van Horne in Outremont).
My hubby likes the quiches from Maitre Boucher in NDG on Monkland. They have quite a few varieties that differ from day to day. I don't know how they compare to other quiches cause that's the only place we buy 'em. ;-)
I wouldn't know where the BEST quiches in Montreal are, but you can find notably better than average quiches on the lower floor at Ogilvy's: Cafe Romy.
----- Cafe Romy 1396 Rue Sainte-Catherine W, Montreal, QC H3G1P9, CA