Bb 13sus4, 4th inversion (B# in the bass)
Also written as Bb13sus4/B#.
Notes, from the bass up
B# (bass)GA#D#E#G#
The same notes as Bb 13sus4 (B#, G, A#, D#, E# and G#), rearranged so B# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.