If you're invested in any of the funds in our "Magnificent Retirement Mutual Funds" list, congratulations on owning some of the best managed and top-performing mutual funds. If you are lucky enough to discover our list of Top-Ranked Funds for the first time, it's never too late to start investing with the best, especially when it comes to your retirement.The easiest, most reliable way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. Our Zacks Rank covers over 19,000 mutual funds has helped us identify three outstanding options that are perfect for any long-term investors' portfolios that is retirement-focused.Let's break down some of the mutual funds with the highest Zacks Rank and the lowest fees.If you are looking to diversify your portfolio, consider T. Rowe Price Growth Stock Fund (PRGFX). PRGFX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. This fund is a winner, boasting an expense ratio of 0.65%, management fee of 0.51%, and a five-year annualized return track record of 14.63%.Buffalo Small Cap Fund (BUFSX): 1.01% expense ratio and 0.85% management fee. BUFSX is a Small Cap Growth mutual fund building their portfolio around stocks with market caps under $2 billion and large growth opportunities. With yearly returns of 13.7% over the last five years, BUFSX is an effectively diversified fund with a long reputation of solidly positive performance.Neuberger Berman Mid Cap Growth R6 (NRMGX): 0.6% expense ratio and 0.55% management fee. NRMGX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 10.35% over the last five years.These examples highlight the fact that there are some astonishingly good mutual funds out there. If your advisor has you in the good ones, bravo! If not, you may need to have a talk.Do You Know the Top 9 Retirement Investing Mistakes?Investing in underperforming mutual funds is just one of the key errors that can derail your retirement plans.To learn more, read our just-released report: 9 Retirement Mistakes You Need to Avoid.This report can help you sidestep these costly mistakes and potentially achieve your retirement goals. Get Your FREE Guide Now Get Your Free (BUFSX): Fund Analysis Report Get Your Free (PRGFX): Fund Analysis Report Get Your Free (NRMGX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research