I think UBC (in general) ranks in the top 40 worldwide, but I don't know how Physics specifically ranks.
Visiting will probably give you a good feel of how the graduate students interact. The University is far removed from the city though, so you might not get to really see much of Vancouver in your visit. Vancouver isn't really very "multi-cultural", it's mostly Canadian-Asian communities, mostly from China, Korea, and India. I love it though, because I feel I fit in very well with the young people there, who are mostly born in Canada but have immigrant parents. I also love the food selection -- very good Asian food! Unfortunately, I don't think Mexican food in Canada can compare with California!
I grew up in a suburb outside of Vancouver and attended UBC Physics for undergrad, so send me a PM if you want more specific questions about living there