First of all, I do not want to discourage you, but I am afraid that the majority of Jehovah’s Witness believers are so well trained to not think or ask reasonable questions that it is quite likely you will not make much headway with them. They are fantastically armed against reasonable arguments. To use a rather loaded term, this religious group carefully "brainwashes" their converts so that even the most obvious logical question seems to have no effect. Of them the saying of Jonathan Swift (author of Gulliver’s Travels) is true: "It is impossible to reason a person out of something they were not reasoned into in the first place."
Having said that, I am sure that there are JWs out there who are open to listening and in any case, it is our job as Bible believers to be prepared to answer! The best opponent to the irrational belief that the New World Translation is the best available version of the Bible is information and truth. This view is absolutely impossible to maintain in the light of the evidence. First of all, we ought to be distrustful of any translation which is the exclusive property of one particular religious group. Scholars and general experience tells us that such word done in isolation, without scrutiny from those with whom we do not agree on everything is a perfect hothouse for producing biased and unreliable conclusions. Bottom line, this transation is the exclusive property of the Watchtower Society and is for this reason alone suspect.
Then one must look at the translation. It rates as one of the poorest in quality of all the translations out there for any number of reasons. The most obvious of these is its blatant mistranslation of any number of passages which relate to the deity of Jesus or the Holy Spirit. I suggest you look at the outline and power point at the web site on the Jehovah’s Witnesses. There are many rather obvious mistranslations found in this outline. Let me mention a few.
The most famous, although perhaps not the most egregious is John 1:1. Here the NWT has, "In the beginning was the word and the word was with God and the word was a god. Here, the translators have inserted the indefinite article "a" where there is no word in the Greek. Of course, their apologists have an excuse for this, but to see how poor their argument is, all one must do is listen to the reasoning of EVERY SINGLE other translator. This is an absolutely obvious example of translator bias, not the evidence, causing the translation. Another example is in Colossians 1:15-17. Here the NWT has "The first born of all creation…. By means of him all other things were made. First of all, they subsitute of for over. Colossians 1:15 has Jesus as the first-born over (not of) all creation. If you are not sure about this, you can use a Greek interlinear or a comprehensive concordance to look up the original Greek and the meaning of the original Greek. Also, the NWT has the word "other" in the text, where there is no Greek word to be translated as other. Both of these are blatant examples of changing the meaning of the text so that it will not say what it clearly says. The NW translators are uncomfortable with Jesus being the first over creation, so they make him the first of creation, with zero justification. They are uncomfortable with him beint creator of all things, so they have him creator of all things other than himself. There is literally no justification for this translation. It is a completely transparent fabrication. The point here is that anyone looking at the Greek will reach this conclusion. It is not controversial (except to the JWs).
The NWT has John 3:16 saying that son is the first creation of the Father. Again, the word creation is not in the Greek. For Revelation 3:14 the NWT has Jesus as the beginning of the creation of God, when the actual meaning is the ruler of God’s creation. Again, a simple glance at the Greek word and a study of its normal meaning will show that the NWT is wrong here. There is no doubt about WHY it is wrong. They are trying to create a false idea that Jesus was a created being. There are an unlimited number of examples. In nearly every scripture which supports the deity of Jesus, the NWT has blatantly adapted the translation so as to create the false impression that Jesus was created. The same can be said for passages about the deity of the Holy Spirit, its use of terms for the name of God and more. It is literally true that not a single scholar outside the Watchtower movement has anything positive to say about this translation. The retort of the JWs is that this is because of persecution and because of the falling away of the church from the truth. This is a blatant attempt to focus the attention away from the key question. What is the most reasonable translation of the original Hebrew and Greek of the scriptures. On this account, the NWT loses every single time.
Bottom line, the NWT is an unreliable translation which is affected more by the biases of the translators than by the meaning of the text, at least in places where it impinges on doctrines held by the Jehovah’s Witnesses. Even in places where there is no big issue over the false beliefs of Russel (the founder of the group), the NWT is not a top quality translation by any measure.
As for defending other translations, you will have to ask me which one. There are a number of very high quality translatiosn of the Bible. Which one is "best" depends on the intended use. The best ones are created by committees which include members of a broad range of Christian groups so as to assure a relatively unbiased translation. There is no best translation. A good student will refer to two or three of the best and have their Greek resources in place.