Romans is full of commentaries, being such a central book of Paul's. This book has been the constant subject of discussion for centuries. With the rise of the "new perspective" it continues to have importance for understadning Paul and NT theology, being Paul's most systematic presentation of his thought. Cranfield in the ICC is full and excellent, but was written before the new perpsective came on the scene. Moo in the NIV Application Commentary series is also quite helpful, as is his superb NICNT volume. Michel in German is also quite good, but pre-new perspective. Fitzmyer is solid and Dunn gives a look at how someone in the "new perspective" approach handles the letter. Schreiner's work interacts with the "new perspective" critically. An old classic is Sanday and Headlam. So dive in and see what the discussion is on Romans.
What commentary on Romans have you found most helpful?