Other respondents have already dealt admirably with the issues of translation and accuracy, and I don't have too much to add particularly to what /u/Cameramano

So instead I will highlight some translation issues in the NWT.

Issues surrounding the divine name. The divine name in the old testament is written יְהוִה (transliterated into English as YHWH). This word, when vowel points were introduced to the Hebrew texts, was written with the vowels of the word Adonai, meaning "Lord", which combination leads to the word "Jehovah". While there is still some debate, most scholars agree that the name was probably originally pronounced "Yahweh". Regardless of whether that is correct, "Jehovah" is almost certainly wrong.

However, I don't consider this the main problem with the NWT's use of the divine name. We know that Jews would generally read "Lord" (Adonai) in place of pronouncing the Name, and this habit continued into Greek, thus the use of the Greek word κύριος to refer to God. The NWT consistenly translates κύριος as Jehovah in the NT, which is a significant interpretative decision that, I would suggest, betrays the goal of accuracy.

Secondly, there are passages where the NWT makes deliberate interpretative additions in its translation to serve theological agendas. A good example of this comes in Colossians 1.15-18

I will quote the Greek (SBL edition), then the NWT (from here), then the NRSV

Greek: 15 ὅς ἐστιν εἰκὼν τοῦ θεοῦ τοῦ ἀοράτου, πρωτότοκος πάσης κτίσεως, 16 ὅτι ἐν αὐτῷ ἐκτίσθη τὰ πάντα ἐν τοῖς οὐρανοῖς καὶ ἐπὶ τῆς γῆς, τὰ ὁρατὰ καὶ τὰ ἀόρατα, εἴτε θρόνοι εἴτε κυριότητες εἴτε ἀρχαὶ εἴτε ἐξουσίαι· τὰ πάντα διʼ αὐτοῦ καὶ εἰς αὐτὸν ἔκτισται· 17 καὶ αὐτός ἐστιν πρὸ πάντων καὶ τὰ πάντα ἐν αὐτῷ συνέστηκεν, 18 καὶ αὐτός ἐστιν ἡ κεφαλὴ τοῦ σώματος τῆς ἐκκλησίας· ὅς ἐστιν ἀρχή, πρωτότοκος ἐκ τῶν νεκρῶν, ἵνα γένηται ἐν πᾶσιν αὐτὸς πρωτεύων,

Michael W. Holmes, The Greek New Testament: SBL Edition (Lexham Press, 2010), Col 1:15–20.

NWT: He is the image of the invisible God,+ the firstborn of all creation;+ 16 because by means of him all other things were created in the heavens and on the earth, the things visible and the things invisible,+ whether they are thrones or lordships or governments or authorities. All other things have been created through him+ and for him. 17 Also, he is before all other things, and by means of him all other things were made to exist, 18 and he is the head of the body, the congregation.

NRSV: 15 He is the image of the invisible God, the firstborn of all creation; 16 for in him all things in heaven and on earth were created, things visible and invisible, whether thrones or dominions or rulers or powers—all things have been created through him and for him. 17 He himself is before all things, and in him all things hold together. 18 He is the head of the body, the church; he is the beginning, the firstborn from the dead, so that he might come to have first place in everything.

The main issue here is in verse 16-18, where the NWT consistently adds "other" as an interpretive gloss. Greek has at least two perfectly good words that mean other, ἄλλος, and ἕτερος, neither of which appears here. JW's theological position is that Jesus is the first created being, and their translation here underscores that idea by reinforcing the interpretation that he is not different in kind from other created beings, only in order.

There are other translation issues of the same kind as this second example. John 1:1, Hebrews 1:8 are two further cases of this kind. One might also examine their decision to consistently render σταυρός as "torture stake", based on the pre-existing conviction that Jesus did not die on a cross per se.

A further issue is that Jehovah's Witnesses has generally not disclosed those responsible for its translation. Some of those translators have been identified in a memoir by Raymond Franz, and the level of Greek and Hebrew knowledge is not reassuring.

So, while there are all sorts of translation issues to acknowledge, and I am happy to discuss both translation theory and particular issues, I would say that the NWT has significant problems that undermine the claim to be "the most accurate translation".

Since you brought up the New World Translation, let me start by saying: It's translated and published by the Jehovah's Witnesses, a small sect with beliefs at variance with most Christians. Many scholars and theologians have criticized it for rendering passages in ways unsupported by the original text so that they support the Jehovah's Witnesses' specific beliefs. If this's true (I've never looked at it myself), saying it's the "most accurate" translation would be quite false judging by scientific or historical standards.

That aside, asking about the most accurate translation involves two real questions: (1) What did the Bible originally say in Hebrew, Aramaic, and Greek? and (2) What is the best way to render that in English?

For the first question, let me just point out one thing off the bat: In over 95% of the text, virtually all original-language manuscripts we have agree. Figuring out what the original text was in the few cases of difference is the subject of the science of textual criticism, and I'd really recommend you ask somewhere like /r/AskSocialScience about that because I don't know too much about it. But you can mostly ignore this to start off with, because the vast majority of the text is the same, and most of the differences are more or less inconsequential.

Answering the second question is a fundamental question of linguistic theory. To generalize the subject a whole lot, you can go either with rendering each individual word into its equivalent, or with a dynamic-equivalence rendering each idea. Take Matthew 9:15 for an example. In a word-for-word translation (King James), Jesus refers to "the children of the bridechamber." English speakers can read that, but we don't understand what he meant. So, a dynamic-equivalence translation (New International Version) renders it as, "the guests of the bridegroom." If you've got more questions about the process, I'd recommend asking somewhere like /r/linguistics or /r/AskSocialScience .

For recommendations of specific translations... I've heard that a number of beginning Greek and Hebrew students use the New American Standard version, because it's an extremely literal word-for-word translation. (I've read it myself, and it's fairly readable as long as you're somewhat familiar with Hebrew idioms.) If you want something using more English turns of phrase, I've also heard good things about the "English Standard Version" (ESV), which is still fairly word-for-word; or the New International Version is a widely-used dynamic-equivalence rendering.

Translation is by its very nature an act of interpretation, and so your answer to this question depends entirely on your personal faith tradition. There is no objective answer.

The default English translation for most academics is the New Revised Standard Version.

Any scriptural translation requires choices on three levels: 1). choices of what variations of existing texts accept; 2). choices of English words to convey the Greek or Hebrew meaning; and 3). choices of balance between word-for-word and overall accuracy. These are all things that have objective boundaries but contain subjective elements. Because of this, your question is really unanswerable. Likewise, your brothers' claims are problematic.

I have some training in Greek and Hebrew from my M. Div. degree. I use that along with BibleWorks software, dictionaries, and language conversant commentaries for serious study. I will have handy the following translations: New Revised Standard Version (NRSV), NET Bible, New International Version (NIV), King James Version (KJV), American Standard Version, a Jewish translation, and The Message - for a bit of fun. These give me insight into the currents of translation in the English language - the choices that generations of scholars and theological traditions have made. An understanding of these choices and their underlying reasons is likely more helpful than a "most accurate translation." Also - far more interesting!

You may also be interested in the good folks over at /r/academicbiblical.