I have read dozens of commentaries of Mark, Ronald Kenaghan's is my favorite. I have read it many times and always discover fresh lines for thinking.
I prepare for the sermon series in Mark every week by reading this commentary and "Let's study Mark" by Sinclair Ferguson [[ASIN:0851517552 Let's Study Mark]]. It is my role to come up with discussion questions for the home groups in my church, and this book has been valuable in helping me create questions that are both challenging and based on insightful Biblical background.
Fairly simple commentary, focused on flow of thought and context. Helpful, if not especially profound.