A priori or a posteriori, which one is more reliable?

Neither. It depends on other qualities of the situation.

But if you’re taking a logic test, go with a priori.

If you’re in a real situation where you have incomplete information, you may want to give a posteriori more weight.

Meaningless question. Which is more reliable left or right?

Your usage would dictate which you should use.

“Hindsight is twenty-twenty.” A posteriori is evaluating after the fact. But it doesn’t help predict an outcome of unknowns.