Democrats: It's close between California and New York, but I'd ultimately go with California. Both home to a ton of urban voters and minorities, and have a healthy mix of Bernie supporters and "New Democrat" types, but I think there's a reason national Republicans constantly attack "California values" and whatnot.
Republicans: Probably Tennessee. Has a mix of evangelicals, suburban moderates and Trumpists. In hindsight, I don't why I thought Bredesen had a chance here.
I still would go with New York since the Democratic Establishment is clearly centered there . THEY are just as attacked by the GOP as California. Remember Criz’s New York values attack . For California until recently it was San Francisco Valies.