Paint.net is (at least in our opinion) the simplest option on the list. It still comes with a ton of powerful features like layers, adjustments, and user-created plugins, but is simple enough that complete newcomers can grasp it immediately. The features are across the top, and filters are accessible from a drop-down menu.
Some of the special effects are pretty cool: sharpen, blur, distort, emboss, etc. You can even get some Instagram-esque vintage effects in there for good measure. We don’t want to waste too much time on this one. It’s got a lot of features, an easy interface, and a simple layout. If you’re struggling with others on this list, then stick with the easiest option.