Which app is most associated with sending files effortlessly between Apple devices using Continuity features?

The app that is most associated with sending files effortlessly between Apple devices using Continuity features is Messages. Messages leverages Apple's Continuity technology, which allows users to seamlessly share files, images, and texts across devices that are signed into the same Apple ID.

When using Messages, you can initiate conversations or send files from one Apple device, such as an iPhone, and continue that conversation or access those files from another device, like a Mac or iPad, without losing context. This makes it particularly convenient for sharing quick updates, images, or documents in a familiar chat interface.

In contrast, other options like Email and Mail are primarily designed for sending larger documents and messages but do not provide the same level of instantaneous file sharing as Messages does. Notes is great for jotting down ideas or collaborating on notes, but it doesn’t focus on file transfer between devices in the same seamless manner as Messages does. Thus, Messages stands out as the primary application for effortless file sharing through Continuity features among Apple devices.