This is part of a larger project reviewing commentaries on each book of the Bible. Follow the links from that post for more information on the series, including explanations of what I mean by some of the terms and abbreviations in this post.
Timothy Ashley's NICOT (1993) is an excellent commentary by a fairly conservative evangelical. It has more detail than some of the older commentaries in this series (e.g. Wenham on Leviticus, Craigie on Deuteronomy), though not quite as much as some of the massive two-volume works in the series (such as Waltke on Proverbs, Block on Ezekiel, or Hamilton on Genesis). I have read this commentary in its entirety, and I enjoyed it very much. The most unfortunate thing about it is that it was published just after Milgrom's JPS commentary (see immediately below) came out. Ashley had access to Milgrom's published papers on Numbers but not the commentary itself. He had enough time after its publication to mention his regrets about this in the introduction but not enough time for it to affect the body of the work. Still, Ashley handles well the historical, theological, and linguistic issues that arise in this book. He tends to avoid authorship issues but treats the book as a unity.
The NAC by R. Dennis Cole (2000) is more recent than Ashley's, but I've heard more mixed reviews. Cole interacts with the scholarship a little more than some volumes in this series, giving plenty of citations of other authors. He argues that Moses is largely responsible for the book. Cole has received favorable comments from reviewers on his handling of theological issues and his analysis of the unified structure of Numbers despite the variety of material in the book. Some of his critics find him somewhat less helpful in biblical theology and narrative criticism. He sometimes spends time on literary observations without making any connection to the interpretation of the book or its theology. Some reviewers consider Cole a better first-choice evangelical commentary than Ashley. Cole does have some stronger points than Ashley, but Ashley is a bit more detailed (although some might prefer a little less detail). What clinches it for me is that I haven't seen the kinds of complaints about Ashley that I've seen about Cole, and thus Ashley gets the nod for my first choice.