powerful linux distribution
Of the most popular Linux distros, I would say Ubuntu is the most flexible, and the best for someone who really wants to learn about Linux
For someone who is looking for a tool and doesn't want to spend a lot of time learning how the tool works, I'd recommend SuSE as having the more intuitive and consistent user interface, and Red Hat for being the best tested and integrated. New features tend to appear first in SuSE, but Red Hat does a lot of work to insure things work smoothly.
As someone else said, it really depends on what you're trying to accomplish.
On the other hand for a quick boot to learn Linux, Knoppix is fantastic.