System fonts are often overlooked, but they come with a massive list of advantages: environmental, performance, data cost.

using system fonts like Arial and Times New Roman instead of web fonts cuts down on HTTP requests, reducing the amount of data transferred and ultimately reducing CO₂ emissions.

Byte-for-byte, JavaScript is more expensive for the browser to process than the equivalently sized web font

The top 5 fonts given by Iain are: Georgia, Charter, Palatino, Hoefler Text (Apple only), and Segoe UI/Roboto/"system" i.e. the main UI fonts for each OS:

font-family: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Helvetica, Arial, sans-serif, Apple Color Emoji, Segoe UI Emoji;
/* Only supported on Chromium-based browsers and Safari */
font-family: system-ui;