I tend to use these five media sites for different reasons and purposes, and to achieve different goals.
I often start with Creative Commons Search because it has the best filters. The visuals here can be used for a range of purposes, such as photographs for themed slides but also complex visuals for teaching.
I tend to use MediaWiki Commons for a specific image, like when I need a simple image of a ladder, or a sample “Spot the Difference” image. You often get a kind of “platonic” version of a thing when you search here.
I have used Flickr Creative Commons for historical images and pictures of real people doing real things. These are scenario or action visuals that can be used to analyze for details and promote visual thinking.
I use Unsplash and Pixabay for theme and title images for slides and web pages. The photography is attractive and high quality. One just has to be careful not to mine them too often because similar pics can come up, depending on the theme.