Here are four resources I use to pick commentaries, I have found them very reliable.
- Ligonier Ministries – In their recommend reading there is a great section on commentaries that helpfully give the difficulty level of each. Also Keith Mathison has been doing some great blogging concerning his favorite five commentaries for various books.
- New Testament Commentary Survey by D.A. Carson
- Old Testament Commentary Survey by Tremper Longman III – I like the format of this survey better, it rates the commentaries with a star system.
- Monergism has started a new website reformedbooks.net and it has a section on recommended commentaries.