If you can only afford one lens you should not be a photographer.
The lenses you choose depends on what you shoot, where you shoot, how you shoot and the camera. Plus your budget.
On digital though you need to decide on one lens as changing them over is a nightmare with dust.So do you want the best photo? Go for a fixed lens that fits where you shoot, I used to use a 85 mm. I think it was an 85 mm.
For digital it's a pro wide angle to around 150-200 mm.
Not sure about the exact one as I'm at home and anyway I'm a pornographer not a photographer.
