I hear "all the time" people saying my favourite lens is ... and then they explain, it's the sharpest, it's the cheapest, it's the lightest...
No it's not, these are just the rationalizations. The real reason is that this lens is the one match their vision. Some people are "wide angle" people, some people are "tele" people...
My favourite lens is the Canon 70-200 f/4 IS. Actually that's one of the main reason that I switched from Pentax, with its vastly superior ergonomics, to Canon. It's half of the weight of the 70-200 f/2.8 IS and even that is starting to get too heavy for me, especially when I have to hold the camera/lens combo with just one hand because I have the flash in my left hand.

Sarah and Salem click on image for gallery