Probably Florida for me. I liked ScubaDada's post about Boynton Beach; haven't been there, but the logic sounds compelling. I like a number of places, but a few thoughts following up some issues:
1.) Are we talking best place solely from a diving perspective, or do cost-of-living, topside activities, etc..., factor in? After all, the question was about the best place for a diver to live, not just visit.
2.) Wouldn't be the Great Lakes for me. Yeah, it's got wrecks. So does the ocean. In the ocean, I can enjoy diving around the wreck, and also see varied, interesting life; a Goliath grouper, barracuda, an octopus, sand tiger sharks, etc... In the Great Lakes...well, what life are you going to see? Does it come close to comparing?
3.) Cold water vs. Warm water. I don't know off-hand if anyone prefers cold water so much as dives it because it's local &/or cheaper for them, or has something the regional warm water doesn't (e.g.: kelp, maybe seals & sea lions). I loved my trip to California this summer; highly recommend it. But I didn't go because it was cold; I went despite the cold, and was blessed to experience other things. Cold means more exposure protection means more weights means more hassle factor.
4.) I don't want to live on an island. I'm not a frequently wide-ranging land traveler, but time spent on Bonaire trips convinced me I like a lot more land mass around me than that.
5.) Good shore diving sounds awesome. California might just carry the day for that reason alone, assuming Blue Heron Bridge & Lauderdale-by-the-Sea might get old eventually? What other shore diving action in southeastern Florida?
6.) If the diving's to be by boat, let it be plentiful and cheap. Rainbow Reef Diver Center in Key Largo did well with that (cheap diving, 2 2-tank boat trips/day) for me a few years back. I'd like to be driving distance from Key Largo if possible. Is there any comparable '4 boat dives/day' option from Jupiter down through West Palm Beach & Boynton Beach? (Note: Emerald Dive Center does 3-tank boat trips, but at least some of that's shark feed diving, which not everyone's into).
7.) Florida's caves mean little to me, but lushly vegetated freshwater springs are of interest. I want to see some wildlife, and Florida's got a lot more diversity than I'm used to seeing at the local quarry. Of course, not everybody wants to see an alligator outside the zoo...
Richard.