You can find a lot of articles about the most successful startups / companies in 2019 but these lack information of one-man companies, unless you started as a one person and now you have dozens of employees.
My candidate is my friend. He built mobile app that generates revenue around 30 - 50k $ per year.
Lynne Tye of Key Values. She made about $400k in 2019 from a site that connects software engineers with companies that that share their intangible values, e.g. diverse team, good for parents, fast or slow-paced, etc.
Robert James Gabriel of Helperbird. He struggled a lot with dyslexia growing up, and even had a teacher tell him he should give up and drop out of school. Luckily another teacher encouraged him to learn to code, and he's been quite prolific since. Helperbird is a browser extension that helps others with learning disabilities browse the web easier. Robert recently brought on a co-founder, but he'd grown the app to a "comfortable five figures a month" in revenue.
Plenty more on https://www.IndieHackers.com sharing their stories via interviews and on the podcast, and also posting about hitting revenue goals and other milestones here: https://www.indiehackers.com/milestones