I’ve tested a ton of epub cover converter plugins. My absolute favorite is 'Calibre'—it’s free, open-source, and incredibly versatile. You can tweak every detail of your cover, from font styles to image placement, and it supports multiple formats like PDF, MOBI, and EPUB.
Another great option is 'Sigil,' which is perfect for those who want more control over HTML and CSS elements in their epub covers. It’s a bit technical, but once you get the hang of it, the results are professional-grade. For a simpler drag-and-drop solution, 'Canva' with its epub export feature works wonders, especially if you’re not tech-savvy. Lastly, 'Adobe InDesign' is the gold standard for professional publishers, offering unmatched precision and customization, though it’s pricey.