There are many domains that one can refer to here: computers, cars, software, architecture, home entertainment systems, game systems… You name it. Let’s cover the ones I just listed.
Computers: I know that Apple does some amazing industrial design, and I love my iPad 2 and MacBook Air, but I have to say that when it comes to this question I have to be somewhat contrarian: the best computer design ever is the IBM Thinkpad. It is a fantastic package, and made the laptop revolution happen. It’s still incredibly useful and attractive.
Cars: I’m a total James Bond fanboy, so I have to say the best car ever is the Aston Martin DB5. It’s just beautiful from any angle.
Software: Keynote, for OS X. It is one of the most easy to use programs ever and it allows users to communicate their ideas affectively and elegantly.
Architecture: I’m tempted to say “anything designed by Frank Lloyd Wright” because I’m a huge fanboy of his work… But I’ll say that Falling Water is the best of the best: http://www.fallingwater.org/
Home Entertainment: I have to say that Sony’s Bravia TVs are just fantastic, the best design for a TV ever.
Game Systems: Nintendo DS and Wii are just really well designed and I have to give the edge to the Wii: it’s ergonomic and interactive and the software and the hardware work together perfectly.