Assume that the state becomes a very large lake, bay, inlet, or other body of water - fresh water if it's inland, salt water if it's coastal.

I'd have to go with Nevada. If the state was too far north, it would create a lot of snow and ice and stuff, which might not be good. Nevada would create a lot of humidity and extra farmland in Arizona, Utah, and eastern California, which would greatly increase American productivity and prosperity.
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