Nope, not murder, adultery, rape, bad as those are.
In my book the biggest sin is to be incurious about the world.
People ask me about what I think about while I'm running hours in the woods alone. "Don't you get bored?" they say.
Outwardly, I say "No," and make some sort of superficial explanation. Little sense trying to convert them. Inwardly I just shake my head and feel sorry for them, that they seem to be that incurious about the world, implying that hours spent out in nature would somehow be empty and bereft of interest and meaning.