It depends whether the choice is limited or not.
You would say, for example 'What colour is your car?' or 'What country would you most like to visit?' because the choice is very wide. There are dozens of possible car colours and hundreds of countries in the world.
However, if there is a limited range of options, known to both the speakers, then you use 'which'. For example, if a husband and wife are buying a new car, they go to the motor dealer's, look at the choice of, say ten colours, which the particular model they've chosen is available in, and they'd say 'Which colour do you you like?' Likewise,if you are watching a sports tournament in which six countries are competing, you might say 'Which country do you think is going to win?'
If you are not sure, use 'what'. It is possible to use 'what' in the second set of situations, but it wouldn't be correct to use 'which' in the first.
I hope that helps.