Two places where if you went straight from one to the other, you’d be shocked it was the same country. It can be language, religion, food, politics, whatever – just the most polar opposite places.
For the US, after thinking, I’d have to say that rural Cajun Louisiana, far from New Orleans or the coast, and Seattle, Washington are about as different as you could get here. One is very rural, traditional, conservative, Catholic, French-speaking, food heavily influenced by African-American and Caribbean culture – the other is a big city full of high tech, very progressive and irreligious, with most immigrant influence (in ethnicity, cuisine, etc.) being Asian. About the only thing they have in common is it’s pretty rainy in both, but even that is different (Louisiana is more “occasional massive storms” vs. Seattle’s “constant drizzle.”)