i've thought about this a lot as well. i come back to probably spain, just due to the language. language aside, i'm almost sure i'd rather live in france or italy.
what regions do you prefer in these countries?
in spain, probably galicia, but maybe las asturias, cantabria, the basque country or catalonia. i think galicia has the lowest cost of living though, and is also right next to portugal. my wife might prefer catalonia, she doesn't like rain as much as me. i could handle that, but if it was just my choice, i'd go for the greener areas.
in france, any of the provinces bordering the atlantic (from brittany down) or the mediterranean (same conflict between myself and my wife) as well as the midi pyrenees and the rhone alps. maybe bourgogne and auvergne (never been to either afaik, but they seem like they might be nice). also corsica, though again, never been there. also not sure how i'd like living on an island. i think my preference would be aquitaine. but honestly, i'd need to spend more time there to know best where to live.
italy is just great, i could live almost anywhere there, although i'd probably steer clear of the islands and the coastal south. maybe liguria as a first choice. but i dunno, i've liked everywhere i've ever been in italy. i'd probably have to look at cost of living before making a decision.