Those cities are big, you would have to ask about specific areas. I live in a very diverse neighbourhood just 10 minutes from Square One (Polish, Ukrainian, Indian, Chinese, Italian, etc) - and we don't have a problem here in terms of crime. There are so many great natural trails here, and it's clean. I go jogging even after midnight.
I hear quite some many areas of Brampton are more mono-cultural that people from other backgrounds won't really fit it.
I don't think any of these cities are particularly dangerous, unless of course, you're involved in dangerous stuff yourself. It just depends on what your preference is. I think the main thing is culture -- there are certain streets that are majority recent immigrants of Polish, or Ukrainian, or Indian, etc origin -- so you might feel "out of place" there unfortunately (even if people are nice). Before you move, just take a walk or drive through the area. Also depends on your type of lifestyle. Toronto and Mississauga are more "happening"/active/busy than Oakville and Brampton (which are more quiet).