I can't pick!
I mean I love my canon 70-200mm f.8 IS L because its the longest glass I own and has a fantasticly fast AF speed - and is also very sharp at the long end (and that is when I am adding in a 1.4TC as well). Great lens!
But then there is my 150mm sigma macro it totally beats the 70-200mm into the ground for closeup work and goes even further into full macro as well. A long working distances makes it a great lens for bughunting - especailly when they are more active and less likley to sit still. Add a teleconverter or diopter and get even more fun and magnification out of the lens!
But wait again there is my 70mm macro which although it has a shorter working distance than my 150mm macro its one of my lightest lenses - making it ideal for using the flash in more creative positions by handholding both the flash and the camera (flash in one hand camera in the other). Its doable with the 150mm, but one gets tried really quick - unlike with the light 70mm. Its also great for those times working indoors or in butterfly gardens where one cannot always step back another pace to get something into focus and the shorter working distance is called for.
And then there is the MP65mm macro and well it does things with macro that the other two lenses can only dream of trying to achive!
So in the end I can't really tell you what my favourite lens is, because I don't have one. I have four lenses where each of which has its own areas where is excels over the others for what it gives me either optically or egonomically.
I can tell you what my least favourite lens is though - the 18-55mm kit lens - I hardly ever use it and when I do its mostly just happy snap family shots (though I did get a neat shot of a robin with it once). Ok I might be being slightly harsh on this lens, but overall its just not a lens that suits my needs on an average day.
(ps I could probably tell you what my favourite lens to own would be - 300mm f2.8 IS L

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