Don't know if it's the most ever, but many years ago a JAL 747 crashed with , I think, 523 people onboard. It was the one where the vertical stab blew off when the aft pressure bulkhead failed. The airplane flew around for a while before loosing directional control and crashing in Japan.
I think it was more or less a "commuter" flight, intra-Japan so they routinely would've had pax numbers like that. Only one FA survived IIRC.
That geocities cite that Brett Hull posted got hosed on bandwidth already, but it said something like 700ish packed across the seat rows with the armrests folded up and the rest sitting or standing in the aisles. The El Al 767s and 757s were loaded similarly, and the IAI Hercs and 135s just jammed 'em in there like LS said.
Reminds me of a show I saw on the Military Channel about the Marine Corps 53s evacuating people out of Saigon (please correct me if I'm wrong on any of that). The footage I saw showed people just packing into the choppers; the pilot would pull the collective a bit to see if it could carry any more, and if it could, he would set it back down and a few more would jump aboard. Nuts.