Palantir Technologies, a defense contractor that uses AI-powered analytics to provide solutions to large amounts of data, has seen a 369% year-to-date return, making it the top-performing stock on the S&P 500 this year. Despite its high market value of over $150 billion, Palantir's quarterly revenue is less than $1 billion, making it the most expensive stock on the S&P by its price-to-sales ratio. Palantir's involvement with Immigration and Customs Enforcement has sparked controversy and criticism from human rights groups. Palantir's CEO, Alex Karp, is worth about $15 billion, while his co-founders Stephen Cohen and Joe Lonsdale also joined the billionaire ranks last month.