That's a pretty hard question to answer as there are so many good places to ride in BC. Where are you visiting from and what kind of riding are you looking for?
Just here in the Sea 2 Sky corridor (North Van, Squamish, Whistler, Pemberton) there are probably more trails than you could ride in a year. If this is your first time in BC, Whistler is a good place to start since there is a lot of variety and it has all the amenities. Maybe add some day trips to Squamish, Pemberton, the Shore, and/or the Sunshine Coast.
Some other random suggestions:
Cumberland on Vancouver Island has a huge trail network and a great vibe.
Revelstoke in the Kootenays has some cool alpine epics.
I also like riding in the Okanagan region because it's so different from the coast. Rolling grassland with fast flowy trails, always reminds me a bit of riding in California.