I'm surprised no one has mentioned Anders Nygren. A Lutheran commentary, so he's not so good on passages like Romans 6. However, there's no one quite like him for getting the flow of Romans. Cranfield is FAR better than Moo, in my opinion, except that Moo does interact with the New Perspective, which arose after Cranfield wrote. But, in my opinion, Cranfield is far better for sorting out the various options and then being in a position to make an informed exegetical choice. You can see the difference particularly in the much-controverted Romans 7, and the infamous "I" passages, where Cranfield has it all over Moo. Cranfield is worth the price of admission for his comments on chapter 7 alone. Longenecker does promise to be a very thorough commentary. I did not like Murray nearly as much as everyone else seems to. Shedd was far better exegetically. I have not looked at Porter yet, but I am very interested to get my hands on it.