I spent the better part of a year in Mauritius back in 2010/2011, and did quite a bit of diving. I remember Passe St. Jacques, in the south close to Le Morne, was best for big stuff. Lots of current drifting through the pass in the reef, which attracted lots of sharks and eagle rays. If I would ever go back to the island, I would head straight there with a WA setup. Corsair Wall in the north was also very good for big stuff (huge schools of eagle rays), but hit or miss. Of course it's been a long time, so things might have changed...
Back in the day there was also the Shark Cave at Round Island off the north side, but I never dove it since the sharks were not there at the time. It looks like they're back though, as tecdivingmauritius.com is offering it as a day trip. Might also be worth looking into!