Simple answer: Ground sesame seeds. That’s it.
Really? Is it that simple? If tahini has just one ingredient, why aren’t all tahinis the same?
Tahini is to sesame seeds as peanut butter is to peanuts. In other words, the way you treat the seeds beforehand, from growing to harvesting to processing, will have a big impact on the final product.
Most tahinis are made with hulled sesame seeds, and this is a good thing, because the hulls can add a sharp, unpleasant bitterness. Some tahinis are made with roasted or toasted sesame seeds, which sounds great, but unless the manufacturer is very careful that all the seeds are evenly roasted (and not overdone), the final product can also be bitter.
And then there is the question of texture. Some brands don’t grind the seeds quite as fine, so the tahini is gritty, not smooth.