Notes are one essential thing to keep in sync, and thankfully there's lots of options available in that department. Evernote is hugely popular and I've given it a fair try, but so far it hasn't quite caught on for me. I'm actually using OneNote right now as it's supported on all platforms I'm actively using and I like the app nowadays. It's quite a turnaround from the crash & burn failure I encountered with an older Windows version a few years ago.
As an avid podcast listener another priority is keeping my podcasts in sync. On the podcasts I regularly listen episodes are typically over an hour long, and as my commute is (thankfully) much shorter than that, I end up listening to most episodes in fragments. That brings a requirement of having my list of unplayed episodes and playback position in sync between my devices. Fortunately Pocket Casts does just that and I've been a happy user for quite a while now.
There's one more app category that would make life painful if things weren't in sync, and that's authenticators for 2-factor authentication. Back when I hadn't discovered Authy, it wasn't only once or twice when I was doing something requiring a login on my phone, only to realise that the phone with the authenticator configured for that account was charging in another room. Now that I have Authy up and running, I don't have to think about possible authentication needs when choosing the phone to pick up.
After reading the paragraphs above it should be obvious why Windows Phone is out of the picture. While there are Windows Phone apps that do more or less the same function as the apps I use on iOS and Android, very few of them sync between the WP and non-WP versions. Microsoft apps plus a few others are a nice exception to the rule, but something like Authy or Pocket Casts don't exist on WP, so that makes WP a non-starter in my current use.
