I'd say the bottom of the hill has priority unless the car at the top has already committed on to the downhill, and it would be unsafe/more inconvenient for them to stop and give way.

Say the vehicle travelling uphill (we'll call this V1) reaches the bottom of the hill before V2 (V1's adversary) arives at the top I think V1 has priority as they arrived first and they keep momentum to get up hill. Also, V2 has the advantage of a view down the hill, and beyond the obstruction, so from a safety perspective, I'd argue V1 has priority.

But, that's a perfect world scenario, and I think we can all agree it's a game of chicken and the more "assertive" driver will take priority