The best lens, made by Perkin-Elmer, is currently mounted on the NASA Hubble Space Telescope. It took 3 years to construct due to its highly precise specifications. The lens surface was polished to an accuracy of 1/20 of the wavelength of visible light, or about 30 nanometres.
A straight forward answer would be the 300mm f/4L IS. Absolutely the sharpest thing I've ever used, but I haven't tried enough lenses in my lifetime so there's certainly much better ones out there. Also, it fits my shooting interests and habits. With a 1.4X TC attached it's great even for faraway subjects. It is my favourite lens.
Remember that everyone will have their favourite lens, but it is not necessarily the best. I feel that the 'best' lens for you is the lens that you will use most often, most effectively and most naturally. It is the lens that fits what you like to photograph, and at a price that you can stomach.
For example, if you do street shooting with an old rangefinder film camera with a fixed 50mm lens and find that it works extremely well for you, then that may be the 'best' for your purpose. If you do sports, perhaps the 400mm f/2.8 is the 'best' thing for your subjects. And if you do wildlife and don't mind lugging some serious mass around, then maybe the Sigmonster 300-800mm is the 'best' lens for your style.
As you can see, the optical quality and prices of all these lenses will vary wildly. What is 'best' for one photographer might be a complete paperweight to another. Although the philosophy always states to buy the best quality lenses you can afford, I also think you should spend the least possible to get the lens that you feel is 'best' for you. It's a compromise between the pocket and the hobby.