(Bloomberg News) The best real estate investment in the past decade was found at the opposite end from trophy resorts and office towers, in 5-foot-by-5-foot lockers.

Self-storage companies, which rent units to small businesses and consumers under names such as "Uncle Bob's Self Storage," produced the best risk-adjusted return among 10 U.S. real estate investment trust indexes in the past decade, according to the Bloomberg Riskless Return Ranking. They had the highest total return and the third-lowest volatility, for a risk-adjusted gain of 10.6 percent. Owners of offices, hotels and warehouses fared among the worst, hurt by price swings.

Public Storage, CubeSmart, Extra Space Storage Inc. and Sovran Self Storage Inc. attracted investors with low debt ratios and steady cash-flow growth in a decade that saw commercial-property values soar to records along with sales of mortgage-backed bonds to finance a wave of takeovers. The debt- to-assets ratio for Public Storage, the largest in the group, is 22.5 percent, half the average 45 percent for REITs, said Michael Knott, managing director of real estate research firm Green Street Advisors Inc., making the stock less susceptible to large price swings if the economy worsens.

"Public Storage has incredibly low leverage compared to the average REIT," Knott, whose firm is based in Newport Beach, California, said in an interview. "It's typically not as volatile."

Warehouses Trail

The Bloomberg REIT Public/Self-Storage Index topped gauges tracking healthcare REITs and regional mall REITs, which returned a risk-adjusted 8.4 percent and 7.5 percent, respectively, in the 10 years through April. Warehouse REITs, which had the highest volatility and the lowest total return during the period, joined hotels at the bottom, with a risk- adjusted gain of 0.8 percent.

Storage REITs release first-quarter earnings this week. Extra Space Storage said April 30 that first-quarter funds from operations rose 41 percent on higher revenue and cost controls. Sovran is scheduled to release earnings after the market closes today, and the other two companies in the group report tomorrow.

The risk-adjusted return, which isn't annualized, is calculated by dividing total return by volatility, or the degree of daily price variation, giving a measure of income per unit of risk. A higher volatility means the price of an asset can swing dramatically in a short period of time, increasing the potential for unexpected losses.

Basic Units

The ranking compares 10 of the 11 property index types within the Bloomberg REIT index. It excludes single-tenant REITs because that index contains just four mostly smaller members whose business of retail leasing is reflected in broader indexes.

Storage REITs had twice the cash-flow growth of REITs in main property types from 2001 to 2011, according to Green Street. Net operating income for storage facilities open at least one year rose an average 3 percent a year during that period, compared with 1.5 percent on average for other REITs.