What's the best 'first prime' for a new mirrorless owner?

Mainstream reviewers and specialist subreddit threads consistently point to the nifty-fifty (50mm f/1.8) for whichever mount you shoot. It's cheap, light, sharp enough for portraits and everyday shooting, and forces you to compose with your feet. A 35mm f/1.8 is the other common recommendation if you prefer a wider, more environmental field of view.