From The Wirecutter:
Here at The Wirecutter, we’ve spent hundreds of hours over the past few years testing hundreds of iPhone and iPad accessories to find the best ones. We’ve also recently tested scores of products in new categories to pick our favorites. You’ll find all these picks, and more, in this guide to the best add-ons for your iPhone, iPad, and Apple Watch. While those devices are our favorite smartphone, favorite tablet, and favorite smartwatch, respectively, they can do a lot more—and they can help you do your favorite things better, longer, and more safely—with the right accessories. (We even have some picks for Apple TV gear.)
Click for the breakdown by category.