We use our free thumbnail generator here at Tubebuddy and also photoshop! To get it to count on YT we recommend the 1920 X 1080 variety thumbnail because that's what YT does. That is what counts in the TB checklist!
Depending on the thumbnail you might not even need any fancy editing, a thumbnail could just be a photo of you in a dynamic pose with the contrast and saturation boosted.
In my opinion, for the best flexibility you want an app that will allow
2, manual cutting and separating of layers (basically background removal)
2, basic effects (such as outer glow, drop shadow, outer stroke) if you can manually adjust the outer glow, drop show etcetera so you can expand out the size and darkness of both the drop shadow and the outline, extend the drop shadow without changing the SIZE of the drop shadow, and the ability to manually change the size and colour of the outline (being able to change the colour of the drop shadow and outer glow also helps a lot)
3, text editing (‘you want an app that allows you to adjust the line space between text (kerning and leading) changing the size and rotation, bonus points if you can apply gradient overlays, but not all apps allow that.
Being able to make manual transparent and coloured gradients on their own individual layers can help a lot too.
4, ability to manually make retangular or circular coloured shapes, the more options the better, but having the ability to use multiple layers for rectangular shapes can help make text stand out better.
Apps are getting better every day, not all will have the above features. But my personal favourites are photoshop (monthly and expensive) or affinity photo (one time payment of around $80 depending on your country) affinity is incredibly close photoshop.
Photopea is an online version of photoshop recommended by roberto Blake,
And of course, TubeBuddy has a thumbnail generator as well,