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You lucky dirty dog.
You can't go wrong in any of those places.
For fall colours, which you'll be hitting nicely, will be best in Algonquin - if you're into that sort of thing.
All are somewhat famous parks, especially Algonquin and Killarney.
All three parks were inspiration places for the famed "group of seven" Canadian painters... So, you'll be happy in any of them.
Killarney has more up and down topography than the others, though the cliffs in Pukaskwa on the shores of Lake Superior are great too.
Will you be camping out? Backcountry camping? What's your skill level?