What conjunction is most often used to join simple sentences that contrast with each other?

The conjunction most often used to join simple sentences that contrast with each other is but.

Here are a few examples:

* The weather was terrible, but the party was still fun.

* I love to travel, but I hate flying.

* She is a good student, but she struggles with math.

Other conjunctions can also be used to express contrast, but "but" is the most common and straightforward.