U.S. exchange traded funds (ETFs) have seen record levels of inflows this year, thanks to a rally in equities and investor inclination toward passive index-tracking funds over their actively managed cousins, as quoted on a Reuters article.According to Refinitiv data, U.S. ETFs attracted a record inflow of $324 billion in the first four months of this year, which was 180% higher than the year-ago period. At the same time, U.S. mutual funds saw an inflow of $318 billion, marking a decline of 58%.A lower expense ratio and tax efficiency are making ETFs more popular than mutual funds. "The expected increase in capital gains tax later this year should result in a higher preference for ETFs versus mutual funds for the highest tax brackets," said Komson Silapachai, vice president at investment management firm, Sage Advisory Services, as quoted on Reuters.As most ETFs are passively managed, there is less amount of buying and selling taking place, which leads to lower capital gains and taxes. Refinitiv data revealed that U.S equity ETFs witnessed a total inflow of $149.6 billion in the first four months of this year, while debt ETFs attracted $283.6 billion, per the Reuters article (read: Capital Gain Tax Hike to Power Inflows: 5 ETFs to Win).Against this backdrop, below we highlight a few ETFs that have attracted maximum assets so far this year. Vanguard S&P 500 ETF VOO – $24.1 billionThe S&P 500 hit record highs lately on upbeat U.S. economic data points and corporate earnings. The U.S. economy grew an annualized 6.4% in the first quarter of 2021, breezing past expectations of 6.1%, following a 4.3% uptick in the previous three-month period. Apart from the reopening-driven third-quarter jump last year, the latest reading marked the best period for GDP since the third quarter of 2003. The labor market has also been moderately steady.Apart from VOO, iShares Core S&P 500 ETF IVV added $10.8 billion.Vanguard Total Stock Market ETF VTI – $16.6 billionThe underlying CRSP US Total Market Index representing nearly 100% of the U.S. investable equity market covering nearly 4,000 constituents across mega, large, small and micro capitalizations. Upbeat U.S. economic conditions have led to such sturdy inflows.Financial Select Sector SPDR Fund XLF – $10.97 billionThe financial sector, which accounts for around one-fifth of the S&P 500 Index, had a decent Q1. Results of the Finance sector have benefited from releases of loan-loss reserves that big banks believe will not be required any more, considering the improving outlook for the U.S. economy.Activity levels in equity underwriting, M&A and trading also hovered around record levels for the seasonally weak Q1, which more than offset the continued softness in lending demand and margin pressures. Plus, a rising rate environment have helped the financial sector a lot.Vanguard Total Bond Market ETF BND – $9.50 billionThe bond ETF fetched about $9.50 billion. Renewed coronavirus threat in different parts of the world, especially in the Asian region, has probably given a boost to safe-haven bond investing.Vanguard Value ETF VTV – $8.82 billionReflationary cues increased benchmark U.S. treasury bond yields in recent months. This has brought back investors’ favor to the value corner of investing. Want key ETF info delivered straight to your inbox?Zacks' free Fund Newsletter will brief you on top news and analysis, as well as top-performing ETFs, each week. Get it free >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report SPDR-FINL SELS (XLF): ETF Research Reports VIPERS-VALUE (VTV): ETF Research Reports VIPERS-TOT STK (VTI): ETF Research Reports ISHARS-SP500 (IVV): ETF Research Reports VANGD-SP5 ETF (VOO): ETF Research Reports VANGD-TOT BOND (BND): ETF Research Reports To read this article on Zacks.com click here. Zacks Investment Research Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report