The number of helpful commentaries on Mark (as well as Luke) is significantly less than what we find with regard to Matthew. Still, there are a few that are worthy of note and should be made a priority in the building of one's library.
Once again, I highly recommend the work of R. T. France, The Gospel of Mark, in The New International Greek Testament Commentary (Eerdmans, 2002, 719 pp.). This is simply the best available commentary and should be the first one obtained by anyone intending to teach or preach through this gospel. A working knowledge of Greek is needed to make best use of this volume. As I noted with France's work on Matthew, his treatment of the Olivet Discourse (Mark 13) will not be embraced by all, but I find it convincing.
Although it is now somewhat dated, I highly recommend William L. Lane, The Gospel According to Mark, in The New International Commentary on the New Testament (Eerdmans, 1974, 652 pp.). When I first studied Mark's gospel, I relied heavily on Lane and found him to be extremely insightful. Virtually all comments on the Greek text are restricted to the footnotes.
James R. Edwards has written the commentary on Mark in The Pillar New Testament Commentary series (Eerdmans, 2002, 552 pp.). I've only read small portions of Edwards but he seems to be quite good. The same may be said of David Garland's contribution to the NIV Application Commentary (Zondervan, 1996, 653 pp.). Although I'm not a huge fan of the format of this series, Garland is always good and worthy of study.
Two weeks ago (November, 2008) I purchased Robert H. Stein, Mark, in the Baker Exegetical Commentary on the New Testament (Baker, 2008, 823 pp.). I anticipate that this will soon become one of the standard works on Mark.
If one were restricted to purchasing only a few commentaries on Mark, I would obtain, in order of preference, first France and then Lane. If a third is needed, either Stein or Edwards would be good.
There was a time when finding a good commentary on Luke was a difficult task. No longer!
The best and most evangelical of commentaries is the two volume work by Darrell L. Bock in the Baker Exegetical Commentary on the New Testament (Baker, 1994/1996, 2148 pp.). Yes, you read that correctly: 2148 pages! If one would also like a more popular treatment of Luke, designed for those who do not know Greek, Bock has written the volume on Luke in the NIV Application series (Zondervan, 1996, 640 pp.). This is one of the better treatments in this series.
The Anchor Bible commentary series is a mixed bag, but I have found the two volume work by Joseph A. Fitzmyer on Luke to be quite good (Doubleday, 1981/1985, 1642 pp.). Fitzmyer is Roman Catholic, but don't let that prevent you from gleaning incredibly helpful insights from his work.
John Nolland has written on Luke for the Word Biblical Commentary series in three volumes. Unfortunately, I haven't had the opportunity to read much of it. If one is looking for a solid work that doesn't require a knowledge of Greek, once again Robert Stein has come through in his treatment of Luke in The New American Commentary (Broadman Press, 1992, 642 pp.).
If my recommendations are going to be helpful, I suppose that on occasion I need to mention works that are not worthy of your investment. In the New International Commentary on the New Testament published by Eerdmans, there are numerous excellent volumes (such as the one by Lane, noted above). But the treatment of Luke by Norval Geldenhuys, I'm sad to say, isn't one of them (Eerdmans, 1951, 685 pp.). I rarely found much of help in this book, and now with Bock and others having written on Luke, it simply doesn't measure up to what a pastor needs in his preparation.
Eerdmans commissioned a replacement volume for Geldenhuys, written by Joel B. Green (1997, 928 pp.). I would like to be able to recommend this commentary, but I can't. Aside from the fact that Green rejects penal substitutionary atonement (and has likened it to "cosmic child abuse" in his horrid book, Recovering the Scandal of the Cross [IVP]), there is little theological help to be found here.
I was also disappointed with I. Howard Marshall's contribution on Luke to The New International Greek Testament Commentary series (Eerdmans, 1978, 928 pp.). If I'm not mistaken, this was the first in this otherwise excellent series. My objection to Marshall is not that it has bad theology but that it has so little theological reflection at all. The book is more concerned with text critical matters and issues related to structure and form than it is with providing insight into the meaning and application of Luke's narrative. Marshall's commentaries on Acts, the Pastoral Epistles, and especially the Johannine Epistles are quite good, but this one just doesn't provide what I'm looking for in a commentary.
And now on to the Gospel of John . . .