The HTML tag that produces the largest heading is the <h1> tag. In HTML, headings are defined using <h1> to <h6> tags, with <h1> being the largest and <h6> being the smallest. The <h1> tag is typically used for the main heading on a web page, and the other heading tags are used for subheadings and secondary headings. For example:

This is a main heading

This is a secondary heading

This is a tertiary heading

It’s important to note that the size of the headings is typically determined by the CSS styles applied to the HTML document, rather than the HTML tags themselves. The default styles for headings may vary depending on the web browser or CSS framework being used.