What do you want to shoot? Each lens excels at different things, and given that there are dozens of lenses (just Canon branded...even more third party, of course) it's hard to tell you exactly what the best lens is. If you're just looking for a good all around lens, the 50mm f/1.4 that Tyler recommends really can't be beat, especially for the price. But if you told me you wanted to do landscapes, for instance, I wouldn't recommend that lens.
If you have something specific in mind, let us know, if you're just looking for something good all around, a fast 50mm is a really good choice.