Kevin Van
Interface I enjoy:

Windows 7. It is visually appealing and it doesn't stray too far from the "mental model" I had of Windows XP, which I have used for many years. I particularly like the search bar. You can search for practically anything right from the main Start menu which is pretty neat.
Interface I dislike:
Windows Media Player.
There are so many sub menus that it becomes a hassle to get to my media. I find that the UI tries very hard to be "easy" to navigate with pictures and icons but fails horribly. I dislike it very much.