At the risk of sounding cliche, the three skills that have been most valuable are:
1. The ability to build software that meaningfully solves a customer's problem
2. The ability to reach people who have the problem my software solves
3. The persistence to grind until we have a decent product and can regularly reach people who have our problem.
The above may seem like they're outcomes. But understanding problems and building solutions is a skill. Finding and reaching people is a skill. Grinding effectively is a skill.
Every other skill (writing well, communicating well, selling the visions, giving good demos and customer service) is in service of the above.
[1] - https://www.savio.io