Episode 120: Pages 382-399 of volume 6 of the Kodansha publication. NOT the end of volume 6. [source]
Oddly enough, it seems volume 6 doesn't actually end where chapter 120 ends. According to at least three sources regarding the publications that came out in Akira's original source, Young Magazine, the final chapter to ever be released through magazine ends with Kaneda and Kei watching the sun rise over Neo-Tokyo after it exploded once again.
However, in the Kodansha publication, there are still 44 pages left before it ends, and I don't see any references showing this to be the case online. So either it doesn't count as a chapter, or these chapters were included exclusively for the collective volumes that came out afterwards. Probably for the best, as ending it at page 399 would be way too open-ended for a story that has so many twists and turns. Either way, what a wild ride this whole manga has been. MILES ahead of the movie, and that's really saying something since the movie was beautiful in its own way.