I was thinking about this and went back and found an old thread that was if you could have one lens what would it be. This is a little different perspective and simply what is your favorite lens and why?
My opinion has changed a lot since I originally got my dSLR, but at this point I have to say...
Lens: Nikon 105 F2.8 VR micro
Why: It is sharp! It is fast. It is versatile for everything from macro, portraits and just as a medium telephoto lens. I can walk around with this lens on, shoot just about anything I want and never be disappointed with the results. It has all the features and is the cheapest lens with all of them...VR, IF, Nano coating, AF-S (SWM).