I will preface this by saying that I am absolutely NOT in the Apple ecosystem beyond an iPhone and Apple Watch. I rely on google for personal and Microsoft for business.
That being said my answer has to be iMessage. If I’m being honest, I get a better experience from my Note 8 and Android with the camera, Samsung Pay and google drive/photos. My car doesn’t support Apple car play or android auto so those don’t matter to me. Two things really keep me attached to an iPhone, iMessages and updates.
Edit: not sure where this would fall, but I also love AirDrop. I miss it when I’m using an android device.