I think it would be far too big a topic to put into one thread.... Assuming you are diving "here" you might want to start with how far you want to drive. Eastern Lake Ontario and the St Lawrence would be comfy day trip to get there. Tobermory is more like 12 - 14 hours as would be Milwaukee etc.
I've covered most of the areas you're likely going to want to dive, so maybe I can take a crack at a summary...
St Lawrence - Several recreational dives from schooners to lakers. Strong currents, mediocre vis in the summer, but wetsuit warm. Massive zebra mussel infestation, fair number of fish
Eastern Lake Ontario - Similar to above but many of the best wrecks are a long way out. Warmish in summer. Major zebra mussel issue, water gets kinda stinky in summer
Western Lake Ontario... Minimal offereings, skip it
Lake Erie.. shallow in close wrecks, crap vis, zebra mussels, lots of fish. Further out (like 40 miles around long point), excellent wrecks but tech depths. Possibly better accessed from US side. (See Jim's post above)
Lake Huron... Tons of great options, primarily from the US side. Clear water, colder, very exposed to weather. Aplena area especially nice
Tobermory - Amazingly clear water, cold, dedicated marine park, cool little down. (I'm biased, I have a place there), Lots of charter options
Lake Superior - Whitefish and Sault St Marie... AWESOME wrecks, but deep and cold, limited charter options
Milwaukee/Chicago - lots of great wrecks, great early season vis., lots of charters
Lake Superior - Isles Royale... great wrecks, clear, cold, far, far away, best accessed by liveaboard (I think)
Honestly, you could spend a lifetime diving the lakes... Hell, I have.
Just come to Tobermory. Keep is simple. This is the Arabia. It's a 7 minute boat ride from my door.