Generally, for an English version, I like to look at
the NET Bible. It has, by far, the most translator notes of any English version, so you can see your translation options as well as the translator's arguments for their choice. It also deals with the major textual critical problems (which means discussion whether a word or passage was original or added later). There are two main downsides, 1) occasionally their theology shines through in the footnotes, and 2) some of their footnotes are overly technical, as they are writing them for a technical audience, not the uninformed masses.
KJV and NKJV are generally sub-par as they base their New Testament translations from the Byzantine Majority Text, which is relatively very late, rather than basing it from an eclectic text which gives weight to older and more reliable texts. For the Old Testament the KJV, at least, is also sub-par, just because of the amount of ancient near east literature uncovered even since the turn of the century has revolutionized scholars understanding of certain words. For example, Psalm 23:4, valley of the shadow of death translation is based off a later Greek translation, because no one knew what the ancient Hebrew word meant (its a little more complicated than that, but thats basically the reason). Now, we know it is highly likely that it means a really dark place.
There are some pretty major translation controversies. The most famous is the passage in Isaiah 7:14, translated in the NIV as "The virgin will be with child", may also be renderd "The young woman will be with child" (RSV) or even "the young woman is with child" (NRSV). This has some major implications for OT-NT prophecy, although I don't know the strength of the claim. Plotinus?
The author of Matthew quotes from the LXX (Egyptian Greek translation of the Hebrew Bible translated shortly after Alexander the Great). When the LXX was translated, the Greek word could either mean young woman or virgin. So, for example, in Genesis 34:3, it refers to Dinah after she had been raped by Shechem. By the 1st century, it could only mean virgin, so when the author of Matthew wrote his gospel, he would have read it as virgin. However, the Hebrew word that the LXX translates from most likely means in its context, woman who hasnt conceived before.