I'm partial to Georgian Bay. It's not listed so I voted for Lake Huron but that's not really fair since the advantages of Georgian Bay do not apply to Lake Huron.
Realize that Georgian Bay includes many recognized diving areas each of which rival those already in this thread. They include among others Tobermory with the best wreck diving services in the Great Lakes (boats and stores) and some excellent wrecks for the advanced diver plus lots of easy wrecks for the newer divers, Midland/Penetang area with lots of excellent shallow wrecks and warm water, Parry Sound with another bunch of good wrecks, Fitzwilliam Island off Manitoulin with some seldom visited and excellent wrecks, Killarney with another 1/2 dozen great wrecks, plus more wrecks in Wiarton, Lions' Head, Hope Bay, Owen Sound, and many more places. There is also some great cave, wall, cavern and rock dives all around Georgian Bay in Tobermory, Giant's Tomb, Killbear and a dozen other places.
Additionally, there are good diving services (boats and air) in at least a dozen different locales around the bay close to cheap accommodations (plus camping in at least 12 different provincial parks).
Diving Georgian Bay can be done on a day basis. That is, it's so close to major cities that many divers drive up for the day.
In my experience there is no other place in North America that offers the variety and quality of diving close to home and with good diving services.
This may surprise some divers but that's because they've limited their view of Georgian Bay to Tobermory. Get out a road map and look at Georgian Bay. It is almost as large as Lake Ontario and has many more rivers, bays and islands than any other Great Lake except for Lake Superior. Georgian Bay has often been called the 6th Great Lake and rightly so.
Manitoulin Island on it's north edge is the largest fresh water island in the world.
My point is that Georgian Bay is NOT Tobermory!