The second worst smell I've experienced on a plane was from an extremely inebriated passenger in a cramped economy cabin flying back from BGI. He was absolutely hammered and smelt strongly of whisky.
The clear winner was a chunky fella flying out to GIG from Paris (again in a cramped economy cabin). His odour was similar to the toilets at the office after a colleague has dried their sweat-soaked cycling gear on the radiator.
I would have loved to have deeply inhaled from a McDonalds bag for the duration of those flights.