From what I have heard so far (for nearly 13 years of living experience in Montreal),
English-speaking institutions:
1) Marioanopolis college (private)--> by far the best
2) Vanier college--> has Science Plus which is an excellent program for those who wish to get in science-related programs later.
3) John-Abbot--> I have not heard much about this school. But it's for sure not as good as the other 2 mentionned above.
4) Dawson--> I don't know much either but I would rank it as the last. Sorry.
French-speaking:
1) Jean-de-Breboeuf--> by far the best among french-speaking institutions.
2) Bois-de-Boulogne--> second.
Myself, I attended Marioanopolis a few years ago and could not say I appreciated this school much but it does help a lot if you want to go to McGill.