That's a tough question because with Linux distro fragmentation, what might be the most polished and best looking may also be the least supported with updates
I'm not a huge Linux guy, but between Ubuntu and Mint which I've used I'm quite partial to Mint's UI. I find Unity (Ubuntu's UI) very clunky and very resource-heavy. Based off of your requests, I think those two are your big options, and from there you can get various flavours to appease your experience visually.