Read the regs....hunting in ON just lists centerfire rifle now. No calibers. - meaning any centerfire cartridge is legal....IF there is a rifle season in that WMU.
You either have a rifle season or don't. Most of southern ON is just ML/Shotgun/Bow
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And now to talk about the topic - 308 is easy, 270 or 243 are also great choices for ON. Agree on 2-XX scopes. Leupold makes great 2-7x33. Burris has a 2-8x35.
Another great calibre for do it all stuff is 7mm-08; can basically take anything huntable in Canada with that one Cal.