if you don't upgrade stamina you will need to find some other way to increase your stamina through eating a ton of food because otherwise you can't catch animals (like the satori mountain event) or climb up towers (the gerudo tower, I think?) or get to certain shrines without a massive headache...
100% stamina
if you're trying to optimize your movement and experience as much gameplay as possible, rush stamina, rush vah Medoh. Outside of situations where you take lethal damage (which shouldn't be often regardless of the number of hearts you've got until you have good armor), hearts are...