What is the best material to use for slide guitar?

With materials for slide in such a wide variety, it can be a little overwhelming on what material is best to choose. Glass and metal are the most common choices, while the most obscure that have been documented are knives, bone, spoons, pipes, and rings. The term bottleneck actually comes from cutting off the end of bottles for alcohol and using that as a slide. Essentially, any smooth, hard surface can being used for slide, so the choices are endless.

So what is the best material for slide? I prefer glass myself, but really that is a choice based on personal preference. I am curious to hear what is everyone's choice on slide? Let me know in a comment below!