Galatians
Like 1 Corinthians, I've never preached all the way through Galatians. My familiarity with the literature, therefore, is limited. Typically I've delved into a variety of commentaries while studying particular verses in Galatians and found the following to be most helpful.
I've always found Richard N. Longenecker in the Word Biblical Commentary series to be very good (Word Books, 1990, 323 pp.). Likewise with F. F. Bruce, The Epistle to the Galatians: A Commentary on the Greek Text (Eerdmans, 1982, 305 pp.).
For those who have a working knowledge of Greek, I highly recommend Moises Silva, Explorations in Exegetical Method: Galatians as a Test Case (Baker 1996 236 pp.). While not technically a commentary, this is an absolutely essential tool for analysis of the Greek text. Silva provides an extremely helpful analysis of the grammar and syntax of Paul in Galatians, as well the literary forms and structure of the epistle, the history of interpretation of the book, its relation to Acts, the controversy over the dating of Galatians, and other important themes, including a section on Pauline theology.
Quite new to Galatian studies is the commentary by Gordon D. Fee, Galatians, in the Pentecostal Commentary Series (deo publishing, 2007, 262pp). Again, I've only dipped my toe in the water, but it felt refreshing! Fee has an especially helpful section addressing the controversial issue of whether pistis Christou is a subjective genitive ("[the] faith[fullness] of Christ"] or an objective genitive ("faith in Christ"). He has persuasive arguments, both exegetical and theological, in favor of the latter (the commentary by Longenecker above views it as subjective).
There are several helpful commentaries on the English text of Galatians. Scot McKnight has written for the NIV Application series (Zondervan, 1995, 320 pp.), but beware of his tendency to interpret the "law" in a way that inclines toward the New Perspective on Paul.
The contribution to the NICNT series is by Ronald Y. K. Fung, The Epistle to the Galatians (Eerdmans, 1988 342 pp.) and is a good resource to keep handy. So too is Ben Witherington's, Grace in Galatia: A Commentary on St. Paul's Letter to the Galatians (Eerdmans, 1998, 477 pp.).
Two more pastoral volumes are by Philip Graham Ryken, Galatians (P & R, 2005, 290pp), in the Reformed Expository Commentary series, and Leon Morris, Galatians: Paul's Charter of Christian Freedom (IVP 1996 191 pp.).
Ephesians
My study of Ephesians has been far more extensive, as I've preached through the book twice in past years and taught it several times in a classroom setting.
Before you even think of purchasing anything else on Ephesians, you simply must obtain the commentaries by Peter T. O'Brien and Andrew T. Lincoln. O'Brien's work is in the Pillar series, The Letter to the Ephesians (Eerdmans, 1999, 536 pp.). Although he interacts extensively with the Greek text, the commentary is extremely helpful to all readers. It is solid theologically and O'Brien argues convincingly for Pauline authorship. Lincoln wrote for the Word Biblical Commentary series (Word Books, 1990, 494 pp.). As with O'Brien, it is technical but still accessible to the English reader. Lincoln denies Pauline authorship, but takes a generally conservative and evangelical approach to the book. Without in any way diminishing the greatness of O'Brien's work, I actually prefer Lincoln when it comes to detailed exegesis.
Two other commentaries are also devoted to an analysis of the original text. Harold Hoehner, one of my former professors at Dallas Seminary, has written a massive book Ephesians: An Exegetical Commentary (Baker, 2002, 930 pp.[!]). I can't read Hoehner without hearing his voice in the classroom, lecturing on Ephesians, back in the mid-1970's. His grammatical analysis is a bit over-bearing at times and it is occasionally weak on theological insight, but this is still worthy of your consideration.
Ernest Best has written the replacement volume for the ICC series, A Critical and Exegetical Commentary on Ephesians (T & T Clark, 1998, 686 pp.). As with Lincoln, Best denies Pauline authorship. Another in-depth analysis of Ephesians is Marcus Barth's two-volume work in The Anchor Bible Series (Doubleday & Company, 1974, 849 pp.). Barth is the son of the famous Swiss theologian Karl Barth. Although not always conservative in his interpretations, he is frequently helpful and occasionally offers brilliant insights. Surprisingly (?), Barth affirms Pauline authorship.
As you can see, when it comes to Ephesians, there is no shortage on technical works. O'Brien, Lincoln, and Best should come first in building your library. But keep your eye out for the release of Max Turner's contribution to the New International Greek Testament Commentary (Eerdmans). I don't know the publication date, but get it!
There are two very good works that focus primarily on the English text. Klyne Snodgrass has written for the NIV Application series (Zondervan, 1996, 384 pp.). Snodgrass is a Baptist and an Arminian and has written one of the better volumes in this spotty series. He devotes as much space to application and the contemporary significance of Paul's letter as to exegetical and theological interpretation. Snodgrass favors Pauline authorship.
John R. W. Stott has once again done a great job on The Message of Ephesians: God's New Society (InterVarsity Press, 1979, 311 pp.). As always, Stott is exegetically sound and has excellent illustrations and applications for today's audience.
When I first preached through Ephesians I thoroughly enjoyed John Calvin's, Sermons on the Epistle to the Ephesians (Banner of Truth Trust, 1973 [1562], 705 pp.). His prose isn't the smoothest, but his theological and practical insights are incomparable. Another great resource for the preacher is Martyn Lloyd-Jones's, Exposition of Ephesians, in eight volumes (Baker Book House, n.d.). Lloyd-Jones is verbose but worth the effort it takes to read him. Non-technical but deeply theological.
Two final commentaries on the English text are Leon Morris, Expository Reflections on the Letter to the Ephesians (Baker Book House, 1994). Similar to his reflections on John's gospel, this is a solid work by a solid and spiritual NT scholar. Designed for the English reader, it is less a verse-by-verse commentary and more a collection of theological and sermonic observations. Walter Liefeld has written for the IVP New Testament Commentary Series (IVP, 1997, 178 pp.). Like most of the volumes in this series, comments on the Greek text are kept to the footnotes. The commentary itself is only @ 140 pages in length. It has occasional good insights, but if you purchase only one commentary on the English text, let it be Stott's.
Although not commentaries, two works by Clinton Arnold are worthy of note (and purchase). Powers of Darkness: Principalities and Powers in Paul's Letters (Downers Grove: IVP, 1992) is an easy-to-read treatment of the role of the demonic in Paul's letters. It contains numerous insightful comments on the nature of spiritual warfare, then and now. A somewhat more technical work that focuses on the nature of power and spiritual warfare in Ephesians alone is his Power and Magic: The Concept of Power in Ephesians (Grand Rapids: Baker, 1989, 1992).
Under no circumstances waste your money on E. K. Simpson's volume in the New International Commentary on the New Testament series (Eerdmans, 1973 [1957]). The only reason you might be tempted to purchase it is because it is bound in one volume with F. F. Bruce's commentary on Colossians. Simpson's prose is elaborate, flowery (is that a word?!), and illustrative (and at times downright annoying), while his exegetical analysis is largely non-existent.