Vx or Vy
For the ab-initio pilot to be taught the climb at best rate (Vy) makes most sense. They are learning a skill and will be expected to be able to fly their aeroplane at its 'best' later. Best angle, although rarely much different in small low powered aircraft should also be practised as a skill.
What you fly,Vx or Vy, as has been said, dosn't really matter save for engine management and care and to, somewhat obviously, avoid hitting anything