Energy issues gained favor with traders in exchange-traded funds in May. Every such fund moved up in the rankings in the Street.com Ratings' monthly summary of the most popular ETFs, as measured by average daily dollar volume of trading.

The

Energy Select Sector SPDR

(XLE) - Get Report

moved from the seventh spot in April to the number four position on the accompanying table of the 20 top ETFs in dollar value of average turnover.

HOLDRs Oil Services

(OIH) - Get Report

moved from ninth in the April ranking to the seventh spot, while

The United States Oil Fund

(USO) - Get Report

jumped from the 19th position in April to number 12 in the rankings.

The

ProShares Ultra Short Oil & Gas ETF

(DUG) - Get Report

joined the top 20 list as number 19, a promotion from the 27th position a month earlier.

Joining DUG as a new member of the top 20 was

HOLDRs Retail

(RTH) - Get Report

, which moved up one position from its April ranking to the 20th and final position on the list.

Displaced from the April top 20 was the

iShares Dow Jones U.S. Real Estate ETF

(IYR) - Get Report

, which moved down one position from its previous ranking of number 20. Also absent from the April roster is the

ProShares Ultra Short Russell 2000

(TWM) - Get Report

, which sank from 18th a month ago to 24th in average daily dollar volume of trading.

Interest in the largest

Nasdaq

-listed stocks picked up in May. Average dollar volume of trading in the

PowerShares QQQ

(QQQQ)

, which tracks the Nasdaq-100 index, swelled from an average of $5.38 billion per day in April to $5.93 billion per session in May.

The leveraged version of the NASDAQ-100 gauge, the

ProShares Ultra QQQ

(QLD) - Get Report

jumped from the number 15 position in April to the 11th spot in May while its leveraged "inverse" sibling, the

ProShares Ultra Short QQQ

(QID) - Get Report

, eased from number eight in the April rankings to ninth in May.

Fourteen of the 20 most popular ETFs achieved positive total investment returns in May, the ProShares Ultra QQQ leading the pack with an advance of 11.94% -- virtually double the 5.93% gain of the non-leveraged PowerShares QQQ. The Ultra QQQ's mirror image fund, the ProShares Ultra Short QQQ, tumbled just about the same absolute amount as the QLD advanced. QID was the biggest loser among the most top-traded ETFs in May, off 11.41% for the month.

The estimated average daily dollar volume figures for the month are computed by multiplying each fund's average daily share turnover by the geometric mean of the fund's April and May closing prices.

Richard Widows is a senior financial analyst for TheStreet.com Ratings. Prior to joining TheStreet.com, Widows was senior product manager for quantitative analytics at Thomson Financial. After receiving an M.B.A. from Santa Clara University in California, his career included development of investment information systems at data firms, including the Lipper division of Reuters. His international experience includes assignments in the U.K. and East Asia.