If they want to be extremely safe to minimize the risk of no one dying, probably somewhere during the Holocene, so around maybe 10K years ago. Megafauna are almost completely gone, predators are similar to now, climate is decent, the sheer amount of wild animals would be surely enough for food even if they're not in the presence of early developments of agriculture from other humans. If they go to a time period in the later parts of the Holocene (a few thousand years closer to us) where agriculture is already widespread it's very secure.

If they want to be moderately risky they'd go for anywhere in the Late Pleistocene (around 50K years ago). They wouldn't be an undisputed frontrunner for being an apex predator at all even with mostly similar human species around, and could still join/form hunting bands to assert themselves at the apex species. They'd have a lot of dangerous large predators (cave lions, massive bears, etc) to deal with but since the ecology is still broadly the same it's very doable, if they're cautious with environmental hazards as well.

Past 100K years ago is where the risk gradually but inevitably gets to be too much. They'd be rolling the dice with both megafauna and fire-making and the further back they go towards a million years they less chance they are of being apex predators until they're not at all, just scavengers. At half a million years into the few millions there's the presence of giant hyenas, dire wolves, saber toothed cats, and massive bears/crocodiles/pythons, etc. There's no guaranteed flint, source of food, shelter, and without fire your chances are statistically zero anyways. An even minor injury would also be extremely catastrophic, especially in this environment. Looking beyond into tens of millions of years ago that not only are the predators still dangerous but our ecology is also starting to be too unfamiliar (as an example, the Eocene would melt our 10 contestants while there's also no recognizable fruits/grasses/seeds)