I was asked today what was the most important accessory to have as a professional photographer. They were expecting an answer like external flash, or tripod. My answer - a lens cloth. No particular brand, or even size. I have a random collection that I keep stashed all around. In the armrest of my jeep, every camera bag, laptop bag, inside the zippered pocket of my camera strap, and pretty much anywhere else I might find nearby when using a camera. Most of the time I have a small one tucked into my pocket, and I hate carrying a lot of things in my pocket.
This is my hands down pick for the one item you should always have with. you. Is there any such thing as a lens that is too clean? They are small and cheap, so you have no excuse not to have one handy.