Actually 2 CheckOut is an electronic gateway and provider, similar to a third party processor so I would not put them in the same boat as a merchant account processor.
I would have to say that LinkPoint is ranked very high. While a lot will disagree, if you look at the Nielson report for the best selling ISOs, Chase was number one ISO while First Data, Paymentech, Wells Fargo, Sun Trust and Cardservice were also in the top 20. They all offer the LinkPoint gateway, often branded as YourPay. Those five made up did half of the volume in processing. And they are all partnered / owned by First Data. Which actually authorize.net uses quite a bit as the transaction processor.