Centering modals that aren't fullscreen is generally the most effective approach. On mobile, aligning modals both vertically and horizontally in the center helps them pop, making it clear that they demand user attention. For desktops, while it's still good to center the modal horizontally, you might want to position it slightly toward the top.

This isn't just about aesthetics; it also ensures that the modal stays put even if the browser window gets resized. Keeping the modal stable contributes to a seamless user experience, reducing any possible friction or confusion.