For reaction images, a well made gif will always be best. A unique jpg slays too, but there's a risk of them being too meme, and PNGs (here meaning transparent image), although I love them are awful for reaction images. They're just really corny. They only work sometimes.
They all have their specific purposes especially when it comes to image compression: JPG for default static images, PNG for transparency effects, and GIF for animation. I'm team PNG!
