For stock investors and traders, what is the most important skill you have to master?

1. Discipline to follow your trading plan/strategy, through the good/bad/ugly.

2. Knowing when to trade, when not to trade (only if your criteria are met).

3. Not being emotional, how do you feel when a trade moves against you?

Very few people can master these, as making good decisions is far more difficult than lots of emotional decisions.