There is no "one best" app to edit thumbnail for videos. However, there are a number of apps that can be used for this purpose. Some of the more popular apps include Adobe Photoshop, GIMP, and even Microsoft Paint.
When it comes to choosing an app to edit thumbnail, it really depends on your personal preferences and needs. Adobe Photoshop is a great option if you're looking for a professional-grade editing tool. GIMP is a good choice if you're looking for a free and open-source option. And, if you're just looking for a basic editing tool, Microsoft Paint might be a good option.
Of course, there are many other options out there, so be sure to do some research to find the app that's right for you.