Ever since target-date funds were introduced a few years ago, critics have complained that the funds -- a mix of stocks and bonds that are supposed to get more conservative the closer the investor gets to retirement age -- are nearly impossible to compare to one another.

Not anymore, says Morningstar, the biggest and most popular mutual fund research and ratings firm. On Wednesday the company announced its first comprehensive rankings of the funds. Vanguard, known primarily for its low-cost index funds had the best group of target-date funds. Oppenheimer was ranked last. Fidelity, the largest manager of target-date assets, offers two series of target-date funds: its no-load Freedom Funds were rated average and its advisor-sold Freedom Advisor funds were rated below average.

Vanguard's funds are five times cheaper than the industry average, with a cost of just $19 per $10,000 invested. It's also one of just two firms to use mostly index funds in its portfolios. And, Morningstar points out, even though the funds held plenty of stocks, many did better during the crash than more conservative portfolios. For example, Vanguard's fund for investors hoping to retire in 2010 had 55% of its assets in stocks and lost 21% in 2008 -- better than Putnam's version of a 2010 fund, which held just 27% of its assets in stocks, but lost 26%. John Ameriks, Vanguard s head of investment counseling and research, says the strong performance is due in part to its large allocation to Treasuries and other government bonds, which held up well during the crash.

Oppenheimer landed at the bottom of the rankings, thanks to a high allocation to stocks and, in Morngingstar's judgment, severe mistakes in the management of its bond funds during the crash. Investors in its 2010 fund lost 42% in 2008. Oppenheimer also had the second-most expensive family of target-date funds, with an average of $173 in fees for every $10,000 invested. An Oppenheimer spokesperson acknowledged the firm s difficulties, and noted that the firm has replaced some of its fund managers and boosted the overall allocation to bonds on the target-date series to make the funds more conservative.

While Vanguard s cost advantage and Oppenheimer s fixed-income troubles won t come as news to savvy investors, Morningstar s new rankings still matter. Advisors, 401(k) plan administrators and individual investors all look to the firm for investing guidance; studies have shown funds with strong Morningstar rankings receive the lion s share of investor dollars. And lately, investors have gotten very interested in target-date funds. In the last five years, they ve become one of the most popular investments, and they re on pace to gather $60 billion this year, a more than tenfold increase over 2003. They re also especially popular in 401(k) plans, where they have been designated as an appropriate default investment by the government.

But as fund companies have scrambled for a piece of the target-date pie, the products have gotten increasingly complicated, and comparisons have become more difficult. Among portfolios with the same target-date, the allocation to stocks and bonds could be radically different, and some funds began to include alternative and so-called hedging strategies. The quality of the underlying funds varied, as did the pace at which the allocations would change. Critics have complained that choosing a target-date fund often involved more finger-crossing than research.