Leave out any time period and region that has head hunting or liked building pyramids of heads, even fresh corpses would be unusable.
Your best time would be Napoleonic era or similar with huge amounts of men dead with less disabling wounds. Earlier times perhaps those despatched by horse archers or similar.
Best of all would be one of the numerous armies that perished through dehydration, starvation, plague or all of the above.
Really you're looking for scenarios where the bodies aren't badly damaged. So an army that left a large amount of plague deaths would be ideal. And realitively fresh corpses either in terms of time or environment.
Serendipitously I believe several armies lost a lot of men to starvation, freezing or disease in history while passing over mountains which would make the corpses last longer both because of the cold and because they'd be harder for predators/scavengers to get at.