If you're heavily invested in the iOS ecosystem and don't want to bother with transferring your data (contacts, calendars, etc.) over to Android/Windows, then get an iPhone 5s.
If you absolutely want a larger phone, then go with the HTC One. I had the previous HTC One (M7) and absolutely loved the build quality. Maybe it's just me, but I've noticed that all Samsung phones have a slight green tint that gives me a headache whenever I look at it. Maybe it's the OLED or maybe something to do with the screen calibration, but I would stay away from Samsung phones.
Also, one major difference between iOS and Android is that if an app crashes on iOS, it doesn't crash the entire phone. I had an app that crashed on my HTC One, and the whole phone crashed.
I haven't had much experience with Windows phones, so maybe someone else can chime in on this.