NEW YORK (CNN/Money) - Retailers don't get much respect. Investors have historically regarded them as mediocre businesses that can't sustain high returns.

The conventional wisdom is that retailers lack the competitive advantages -- such as unique technology or patent protection -- that prevent rivals from undercutting them on price.

Recently, however, many investors and analysts have turned bullish on top-quality retailers -- such as Home Depot, Wal-Mart and Walgreen -- because the chains' sales gains have been so strong in the past few months.

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Other analysts say that consumer spending is running out of steam. I don't agree. But in any event, both sides of this debate miss the bigger long-term story.

Contrary to conventional wisdom, the best retailing stocks have been able to show remarkable consistency, churning out above-average earnings gains for more than a dozen years. These gains are possible because the very largest retailers do, in fact, develop competitive advantages.

Retailers may not have the patent protection of pharmaceutical giants or the technological edge of leading computer stocks. But the largest chains are able to wring significant discounts out of suppliers. And increasingly, retailers are developing high-tech computer systems to monitor buying and inventory management.

The advantage that comes from modern retailing information systems is likely to grow, since only the richest, most powerful retailers can afford heavy capital spending right now.

Here are three top-quality retailing chains that have posted higher earnings per share for at least 15 years straight.

Home Depot (HD: Research, Estimates) is the top retailer for do-it-yourself home improvement with more than 1,700 warehouse-type stores. The company plans to open 175 new stores over coming year. Home Depot is also buying back stock to bolster per-share earnings growth.

Home Depot's earnings have been rising at a 13 percent annual rate over the past few quarters. Results for the full fiscal year are scheduled to be reported on Feb. 24, and analysts are looking for even stronger numbers.

For the coming year, earnings gains are estimated to run about 14 percent; earnings growth is expected to average 12 percent to 13 percent annually over the next five years.

At $36.30 a share, the stock trades at about 18 times earnings for the coming year.

Wal-Mart (WMT: Research, Estimates), the world's largest retailer, has more than 3,500 stores in various formats worldwide and plans to open more than 400 new stores over the coming year.

The company has been posting unexpectedly strong sales gains in the past couple of months. Analysts expect impressive gains when the company reports results for its full fiscal year on Thursday.

Earnings are projected to rise nearly 15 percent over the coming year and continue to grow at a 14 percent annual rate over the next five years.

At $57.52 a share, the stock trades at 25 times earnings for the coming year.

Walgreen (WG: Research, Estimates) is a leading drugstore chain with 4,291 stores in 44 states. The company plans to open 450 new stores over the next year for a net gain of 365 after old outlets are closed.

Sales gains have been very strong for the past few months. Earnings for the fiscal quarter ended Nov. 30 (excluding nonrecurring items) were up more than 13 percent.

Earnings are projected to rise 14 percent over the coming year and continue to grow at a 15 percent annual rate over the next five years.

At $34.92 a share, the stock trades at 24 times earnings.

Michael Sivy is an editor-at-large for MONEY magazine. Sign up for free e-mail delivery every Tuesday and Thursday of Sivy on Stocks.