What scent would be good for a diffuser in the living room?

I was thinking of giving a diffuser as a gift to a friend.

My friend said they don't really know and asked for something safe. I think they might get a headache if it's too strong or has a strong scent, but if we keep it in the house, we have to keep smelling it.

I think a subtle scent that everyone likes would be nice.

What scents do others prefer?