I don't suppose there is one all encompassing guitar tuning that is universally accepted as the lowest. There are certainly some songs that spring to mind where the artists and guitarists who have recorded have attempted to really push the limit of the guitar and its pitch capacity, where the tuning reads lowest to highest string pitch: F# - B - etc etc. These are drastic examples, though, for the lowest you are likely to see commonly used on recording or at a gig is B standard, wherein the standard tuning of a guitar is taken and each individual string is diminished in pitch by five semitones, rendering it: B - E - A - D - F# - B.