Guitarist Kirk Hammett from Metallica........?

The guage of string he uses would depend on the tuning of the particular song that he is playing. Metallica most commonly is tuned to standard E tuning so therefore you will want to use 9 guage strings. But they do have some songs such as Sad But True that is downtuned to D and some of their songs are downtuned to C. The lower the tuning you have, the higher guage string you will want. So if you downtune to D, use 10 guage strings, downtuned in C, use 11 guage. The higher the guage, the thicker the string. Now the reason for thicker strings is to keep the tension in them so they don't become loose and have and totally muddy, floppy sound. But overall, if you really want to duplicate Hammetts sound, you need to buy the Randall cabinets and head that he uses with the interchangeable modules in them.