Canva hands down the best. I use it for all my static visual creative needs.
Try to canva
Canva and Crello
Most of the app leaves a watermark.Also on app making a thumbnail takes a lot of time. So I will suggest you to use Canva. CANVA is a online website.Check it ,It will help you to create a lot of things like thimbnail for blog, youtube, Facebook.You can also create channel art ,logo and other stuffs.
Recently, I noticed that many thumbnails of different channels look same. Then I realized, this is the result of using same tool by everyone.
There are lots of tools and Canva is the most popular one. However, I am a fan of PowerPoint and I use it to make my video thumbnails. If you have minimum design sense then PowerPoint would be perfect for you to create your own unique video thumbnails.