Backpacker street with loud bars, scorpion-on-a-stick, and Chang Beer T-shirts. Go once for the experience, then move on.
Nearby alternatives: Banglampu (locals' Khao San), Rambuttri Road (quieter, leafy), Thonglor and Ekkamai (modern, upscale).
Khao San from 22:00 onwards is the peak chaos — go then if you want the full experience.