Philippians is one of the few books that has 3 top-notch, excellent commentaries.
1. The best commentary on Philippians is Walter Hansen's Pillar NT Commentary on Philippians. Readable, pastoral, extensive, helpful, Christ-centered, applicable. Everything you look for in a good commentary.
2. Gordon Fee's NICNT commentary on Philippians is a close second to Hansen's. This is a very good commentary (my favorite in the NICNT series). Fee does a great job outlining passages and keeping the big picture in mind, while at the same time providing rigorous exposition.
3. If you can read Greek, PT O'Brien's NIGTC is excellent.
4. I also like Hawthorne (WBC), but found myself disagreeing with his conclusions more than the others.
6. DA Carson's little book, Basics for Believers, is a great small resource for understanding the main point of each chapter.
7. I think MacArthur's Philippians homeletical commentary might be one of his best.