That's a tough one. The one I personally use the most is my 50mm 1.4 lens. It's worked for me in so many situations...low light, outside, inside, etc. It's a great lens. The 50mm 1.8 is a fantastic lens too, and you can't beat the price (usually around $100). Also, it's not a heavy or bulky lens, and you can sort of make it a macro lens by adding magnification filters. I'm guessing a zoom lens with a wide range and a wide aperture would also be a great all purpose lens, but I find that they cost almost as much or more than the camera body itself. The other factor to consider is what type of stuff you're shooting. I take a lot of shallow depth of field naturey crap shots and the 50mm works well for that. It's also great for portraits (of family, friends, cats...). If you're doing a lot of sports stuff or photos of kids up on stage, something with a zoom might be a better all purpose lens. But! Even then, I think the 50mm would still be a good lens to have, not necessarily for those situations, but just in general.

Hopefully that helps?