Other humans.
Seriously, we are a social animal for a reason. We have always existed in packs and are actually shit at surviving alone. We can do it for short stints and even some longer periods, but the work involved in gathering food and water, preparing meals, maintaining shelter, clothes, tools and everything else is immense. And then there is the whole issue of defense. We have archeological evidence of violent death going well back into the paleolithic era. You're going to face packs of other people who want your stuff. No matter how good you think you are, you're going to get overrun if you are alone. Your best defense is your own tribe.

So ya, if I'm alone in the woods when a nuclear apocalypse happens, I'm going to do my level best to find and integrate into some sort of larger social structure while there are hopefully enough not terrible people around to be able to survive the attempt.