I'm by no means a tartare expert and have no comparison experience, but we did have a good tartare at Marche 27 on Prince Arthur this summer - its across from the Olde Orchard, just west of St. Laurent.
You can have it a few different ways. They also have buffalo and tuna (I think they had horse, but dropped it due to lack of demand).
Also mentioned here
Check out the threads:
Steak Tartar [sic]
Best Steak Tartare?
And, if you can open your zoological window a little wider, make a beeline to Au Pied de Cochon for the bison tartare. (The starter is a more sensibly sized portion for one person than the huge main dish, btw.)
You must try Le Mechant Boeuf in Old Montreal. They even offer a starter dish with 3 kinds of tartar( tuna, salmon and beef) its so delicious! The restaurant is great and very hip.
I can recommend a few places from personal experience...
1. Helm on Bernard
2. La Bete Humaine (though be warned, the service here SUCKS)
3. Lebanese restaurant Daou in Saint-Laurent - technically not a traditional tartare, but their kibbeh nayeh is amazing. This is pounded raw beef mixed with cracked bulgur and seasonings. Utterly delectable.