What is the #1 ETF?

As a financial professional, I'm often asked about various investment vehicles, including Exchange-Traded Funds (ETFs). So, I'm often curious to understand which ETF is considered the "#1 ETF" by market participants. Is it the one with the highest market capitalization? The one with the best historical returns? Or perhaps the ETF that offers the most diverse exposure across asset classes? There's no single definition of "#1," but I'm interested in hearing what criteria people consider when making this assessment. Understanding this can help investors make more informed decisions when it comes to allocating their capital.