TSP Dog? Which TSP Fund Has Highest Returns in 2025 and Why? Everyone’s watching the usual TSP stars, but the real action is with the underdog. Meet the I Fund: the quiet TSP dog making noise I Fund TSP Dog Among Investors The Thrift Savings Plan (TSP) funds are having a good year. While the number of investors in the I Fund is relatively small, this fund has, by far, had the best year of any other TSP Fund. The I Fund is up 22.14% in 2025 as of this writing. The TSP Fund in second place is t...