The Windows 2000 user interface. Uniform look and behavior, very discoverable, clearly identifyable controls, “boring” gray.
Contrast with Windows 11 - inconsistent UI, and adverts and notifications/nags blasting at you from every place they could fit one in. The exact opposite of calm.
iOS multi-tasking by dragging the “home-bottom-bar” is great. Manipulating “cards” of apps with ease and discarding them by swiping them up. It’s very nice, but could be better.