"How accurate is translation X compared to translation Y?" sounds like a simple question, but it really isn't at all. There really can't be a single definitive translation, for a variety of reasons.
Some questions to ask about any particular translation are:
What source texts were being used? Newer translations aren't automatically better than older ones, but most scholars will agree (for instance) that the King James / Authorized Version must suffer from the fact that the translators only had access to relatively late copies of many texts.
What is the goal of this particular translation? The Good News Bible is more about accessibility than accuracy, so while it strives to be a faithful rendition, the need to use a compact vocabulary and keep sentences simple means that some nuances will be lost.
*Are there footnotes showing, for instance, important variant readings?" Translations that aim for high accuracy should also draw their readers' attention to issues such as doubt over the meaning of particular words, cases where there are rival ways of seeing where sentences and phrases start and end (there's no punctuation in all those Greek texts!) and significant differences between source texts that haven't been resolved by scholarship.
There really isn't - and couldn't be - such a thing as a definitive translation - even less a permanent one. Language just doesn't work like that. But it's well worth getting to know the aims, strengths and weaknesses of a few translations, particularly if you want to get to grips with the fine detail.
For me, I most often read Today's New International Version, because I find it reads very naturally, if I'm studying more deeply I'll reach for the *New Revised Standard Version," and if I'm looking to freshen up my view I'll add in a reading with more of the translator's personality, such as Eugene Petersen's The Message or J. B. Phillips. I have a host of others on shelves both actual and virtual, and I have at least enough rusty Greek to put an interlinear (Greek with English) to use if I want to try and see why translations differ.