MacOS — making me go into system settings to allow an app to install that I’ve initiated. I know where the app comes from, I just downloaded it, why do I have to tell my own computer I trust myself to not be an idiot?
iPadOS — multitasking. I know that things are better than they used to be but it’s still a clunky experience to use multiple windows or flip back and forth between them unless you use a keyboard (at which point why wouldn’t I just use a laptop?)
iOS — not a lot these days. I’d still like to be able to customise my Home Screen layouts more, and shortcuts are still pretty unreliable, but I’m largely happy with iOS itself.
watchOS — better customisation for the widgets section. Let me have multiple complication/shortcut blocks, let me organise widgets more easily, etc. More watch faces would be nice too. A lot of the ones that are available are pretty ugly.