I noticed the OP of this thread asked about a Merchant Account. Like others have already said, it depends on your use case and what is most feasible for you. Some of these providers do not permit certain types of activity or sales of products due to levels of risk and other types charge different rates depending upon your usage. Keep in mind that products like Square are not an actual merchant account. That is a payment gateway. You can tell because your business name is not listed exclusively on credit card statements. Instead it might show "Square*YourBusiness" instead of just "Your Business" If you want an actual merchant account, you would have to speak with a place like First Data or PayQuake to name a couple. If you get an actual merchant account, they will usually do a credit check on you and things of that nature and your application gets reviewed by underwriters. If you are looking to do online services, then you may be better off with a payment gateway like 2checkout or paypal. A good merchant account provider for online services would be CDGCommerce.
What is the main difference between a payment gateway and a merchant account provider? Often times it will be the fees. For example payment gateways such as Square or 2checkout often charge around 3.25% per transaction percentage wise plus 20 cents to give you a ballpark idea. But if you have an actual retail store with a terminal and you sell clothing, you have less risk as a brick and mortar store and you can reduce your transaction fee to a rate as low as 1.5% in some cases instead of the 3.25%. So what this really comes down to is whether you have an internet business taking online purchases or if your a person taking transactions from someone in person. The fees are higher for online transactions because there is no physical credit card being swipped, therefor your placed into a higher risk category.