For smart phones, I originally had an Android phone, then went over to iPhone back around the iPhone 4 or iPhone 5 generation, had an iPhone for 2 gens, then went back to Android with the Nexus 6, and then continued with Android since then on the Pixel lineup.
When I switched from Android to iPhone it was just too long ago.. probably like 2010? At the time, I missed things like native sharing to whatever app I wanted to, as opposed to Apple's small list of approved sharing sites. But that's all been implemented into iOS for the most part as the years have gone by.
When I transitioned from iPhone to Android the only thing I missed was iMessage, and mostly, just using iMessage on my computer to text/communicate. There were a few similar services -- Hangouts, Pushbullet, others -- but none worked quite so well. That feature has since been implemented with Android's native 'Messages for Web,' which I like more than iMessage because it's platform agnostic.
Today, if I were to switch, I'd mostly miss Google Assistant / Android Auto. There isn't an iOS alternative that remotely does those core, basic features of those two. Siri is a joke and CarPlay is a nice UI for using media in a car, but it lacks the useful parts of Android Auto which are powered by Google Assistant which is what I primarily value Android Auto for. I'd also miss the basic native implementation of GOogle's ecosystem, which is the ecosystem I use the most. I don't want my primary apps to be in bastardized sandboxes on the OS, although Apple has improved that as the years have gone by, but it still feels that way when I use my iPad. I'll most likely never switch back to iOS unless like Android doesn't exist in 10 years, or something, but I think it's more likely that none of these companies exist at all or phone OS' dont exist anymore than it is likely that I'd be choosing between Android or iOS again.
