If yours is a new business, Square can be the cheapest. And Payment Depot can be the best bet if you want a subscription-based card processing solution. For e commerce merchants, Stripe can be the right choice.
The cheapest card processor may not be the best. You should make a list of features you want, and start researching various card processing solutions that charge low processing fees. Compare different options to settle on the best contender that ticks all the boxes.