iOS’ Font
This may seem petty; that’s because it is. I don’t care for the fonts on Android devices. The stock font looks a bit cheap. Other downloadable fonts are often either too bold, too thin, or just wonky-looking.
Apple has perfected the font used across all of its devices. Apple’s implementation of Helvetica-neue is beautiful and easy to read.
Sure, you can change the font on Android, and it’s really easy to do so, too. However, I searched high and low for a similar font to the iPhone but didn’t have much luck finding a proper one. Just be aware that if you switch to Android, the text might always look just a bit off for one reason or another. Building on Android/Samsung fonts, you won’t like their emojis, either.