Same messaging experience for everyone (including emojis), great desktop app, easy backup & restore as you switch phones, was early on the reply-swipe functionality, easy forwarding, voice messages, . No, none of these are "killer" features, but it's honestly one of the purest examples of 'Just Works'(TM) I can think of in my digital ecosystem.
SMS on Android is pretty crap, and it's a particularly shitty experience communicating with an iPhone user or in groups.
Every time an article about Whatsapp on HN or Reddit pops up I fearfully look to see if there are legitimate privacy concerns. Afaict, all my messages are still E2E encrypted, and all my stuff is saved to my Google Drive.
Unless you're in a sheltered circle of only iPhones (probably in America), Whatsapp is the best choice by miles IMO. (Edit: sure if Apple would democratize iMessage for x-platform I'd consider it, but given that will never ever happy, Whatsapp is the great equaliser)
No other messaging app has the reach and consistency. And yea, this is a hill I'm willing to die on.