Fidelity Investment's robo-advisor was ranked the best in the nation in the second quarter, followed by Vanguard and SigFig.

Fidelity Go was rated the top overall robo-advisor by The Robo Report, a semi-annual report by Backend Benchmarking that uses 45 metrics to track the products, including performance, cost, access to live advisors, transparency and conflicts of interest, the company said.

This was the second consecutive report in which Fidelity Go finished first, the report said.

"Portfolio performance, low costs, and a strong digital-planning platform have propelled them to our top spot," the report stated.

The report said that while Fidelity Go has a relatively high management fee of 0.35%, that is partly offset by the underlying Fidelity Flex mutual funds, which carry zero expense ratios that make "the all-in cost at Fidelity in line with low-cost alternatives."

Fidelity Go also won praise for its hybrid robo solution, called Personalized Planning and Advice, which gives investors access to live advisors for an all-in fee of 0.50%.

Although it lacks some features like automated tax-loss harvesting, Fidelity's platform offers high overall quality, low all-in costs and a strong-performing portfolio, the report stated.

The runner-up to Fidelity, Vanguard Personal Advisor Services, doesn't have many of the techological bells and whistles of its competitors, but its "human-powered advice teams can provide customized quality service at a considerably low price," the report said.

Vanguard's robo-advisor has a management fee of 0.30%, which undercuts the competition when factoring in the access that investors get to live advisors, according to The Robo Report.

"Vanguard is a great choice for those looking for an advisor-centric experience while keeping costs to a minimum," the report stated.

SigFig's robo-advisor was given an honorable mention, with the report calling it a low-cost option that still is able to provide access to live advisors to clients with as little as $10,000 in managed assets.

"To further entice new clients, the first $10,000 in assets is managed at no cost, making them a great entry point to professionally managed money for individuals unable to meet traditional advisor minimums," the report stated.

These were among the other category rankings in the report:

• Best Robo For Complex Financial Planning Needs: Vanguard finished first in this category because of its ability to provide quality live advice to investors for a relatively low management fee, the report said. "For those with complex planning needs, Vanguard is a quality option due to their emphasis on the human experience and their low costs," the report said. The runner-up was Personal Capital, followed by Ellevest and Wealthsimple.

• Best Robo For Performance At A Low Cost: The report ranked Fidelity tops in this category because of its combination of a 0.35% fee and zero expense-ratio funds. Runner-up WiseBanyan was cited for having the second-best performance behind Fidelity and not having a management fee for clients in its most basic service plan for taxable accounts.

• Best Robo For Digital Financial Planning: Wealthfront won this category, followed by Personal Capital and Betterment. "The best digital planning experiences continue to be offered by independent robo advisors," the report stated. "Wealthfront and Personal Capital stand above their peers by offering  robust  planning  tools  to  clients  and prospective clients alike."

• Best Robo For First-Time Investors: Betterment and SoFi finished first and second, respectively. Betterment won high marks for having no investment minimum and a 0.25% annual fee for its digital-only product. "Betterment’s single-goal financial planning tools make it easy for first-time investors to plan for future financial success," the report said.