Since I teach a Sunday School class, I've been asked a few times which study bible one ought to buy; which one is the most accurate translation, has the best notes, etc. Those are tough questions to answer in generalities.
First, there are a number of good translations, starting with the good old King James Version (KJV). Most of the modern version are good as well. But before you can pick a version, you need to ask yourself what kind of philosophical foundation you want in the text. Bible translations are done on one of about three (give or take) foundations- (a) literal, word-for-word; (b) dynamic equivalent (so-called thought-for-thought); or (c) paraphrase. I have condensed these a bit, and there are many ways to make distinctions such that there are more ways to translate the bible than these three, but I've done this for the sake of brevity.
Some paraphrase versions- New Living Testament (NLT), New English Bible (NEB), Today's English Version (TEV), Phillips', Living Bible, The Message. If you want to read scripture like a story, with writing at the 9th-grade level or below, these are your target. You will gain readability, but lose accuracy and detail in translation. In addition, several are the work of single individuals, so you get their biases in the text.
Some thought-for-thought translations- New International Version (NIV), Today's NIV (TNIV), Revised Standard Version (RSV), New RSV (NRSV), Amplified Bible, updated New American Standard Bible. These try to strike the middle ground between literal, word-for-word translations and the paraphrases. On the whole, they are mostly good and useful for most people. If you want precision to the original text, they aren't as good as the next category, but they are certainly better than the first category.
Some literal word-for-word translations- KJV, New KJV (NKJV), original NASB, English Standard Version (ESV). The KJV is the historical standard, in spite of a few translation issues. However, it is written on a 16th-grade reading level, and many people find the language inaccessible. The NKJV is an excellent translation, and I recommend it. The original and updated NASB versions are good as well, though the flow of the language can be stilted in some places. The ESV is one of the newest version, and in my opinion, the best. It is as readable as the NIV, but retains much more accuracy to the original languages, and retains the genders in the original languages.
There seem to be almost as many study bibles available now as there are versions. This is really a bit silly, though in our consumerist culture, I can see why this is so. If your primary interest is understanding the text and the meaning of the original authors, I can narrow the list down to just a few. Here are my recommendations, in order-
(1) MacArthur Study Bible, ESV or NKJV
(2) Reformation Study Bible, ESV
(3) ESV Study Bible, ESV (obviously)
In terms of conservative, gospel focused commentary, I don't think you can do better than the MacArthur Study Bible. Since it is now available in the ESV (as of Summer 2010), it's a no-brainer. Dr. MacArthur provides more meaningful and helpful notes on the text, stays true to the original intent (as defined by historic, post-reformation orthodox Christianity), and is a bona fide biblical conservative, than any other translation out there.
The Reformation Study Bible, edited by R. C. Sproul, is also excellent. It has notes of no less value, but there are fewer of them. It has the advantage of having been compiled by a committee of men rather than a single individual (like MacArthur), which can be important (though not in the case of Dr. MacArthur's work, in my opinion).
The ESV Study Bible is the biggest in terms of the volume of helps and notes. As a result, it is BIG. I keep one of these on my desk for reference, but don't carry it to church. It has a tendency to move a bit further to the left in terms of the notes, compared the the two bibles above, but it is still a very conservative, evangelical (in the traditional sense) study bible. It has the best (most, and high-quality) maps and charts. And if you buy one, you get free access to the online version of the study bible, which I find very helpful.
These are just my recommendations...you may find another that you like better, and that's fine. But if you ask me, these are the three suggestions you'll hear.