I'm endlessly impressed at how good Synology's DSM interface is. The experience is so incredibly seamless in all the important ways. And it's not just a gimmick—all of the benefits of draggable, resizable windows are there: you can drag files between folders, you can open multiple settings windows at the same time, and so on.
There's a live demo: https://demo.synology.com/en-global/dsm