1) The first time I read through it all. I was going to a church that was a KJV-only-or-it's-not-really-the-Bible kind of church. So I as a 14-year-old read the KJV 1.5 times (NT, then OT, then NT again).
2) As the years went by I read different versions and they were interesting, but nothing that was like "Aha! Now I see it 10 times better than before!" kind of thing. Until a friend of mine gave me one that was in chronological order (with notes to why they said this part was written such and such time, but emphasized off and on they weren't sure and could be written at a different time). That gave me a whole new light on things like the meat of Paul's letters (if you know what was going on in the early church at the time from reading what was going on in Acts) and reading the prophetic OT books (again if you knew what was going on at the time by reading 1 & 2 Kings and 1 & 2 Chronicles). It was NIV, but to be honest it wasn't so much the fact that it was NIV that was insightful (I had already read through the entire NIV by then anyway). It was having things in chronological order that was insightful.
Then there was the time I read the entire Bible from Genesis 1 to Revelation 22 in 4 different versions simultaneously (KJV, NASB, NIV, and AMP). Any time I saw a discrepancy among how one of them translated something differently from the others I stopped what I was reading and looked things up in the Strong's concordance and saw both the definition of the original word and also how that same word was used in other parts of the Bible.
Based on that time I did that I'd have to say the Amplified is the most accurate. It was rare I'd find something the others "got it right" and was different from the Amplified. Every now and then the King James Version was the most accurate (most of those times it was in the Old Testament).
IMHO, the NIV wasn't so bad off it changed any of the meat to any of the truths of the Bible. The only times it's inaccurate is if you're wanting to dig into the weeds for specific details that, to be honest, sometimes weren't meant to be focused on to begin with. Like us trying to count acorns on the ground when God wants us to see the forest right in front of us. I gotta admit I often fall in the camp of overanalyzing the Word. :)
Square brackets mean a word or phrase my be translated incorrectly -- they're letting you know they're not sure. Look at Deuteronomy 6 where in verse 4 it says "Hear, O Israel! The Lord is our God, the Lord is one [the only God]!" That's their way of saying, the Hebrew word "echad" means "one" or sometimes "only one", but the translators aren't sure which for that verse. I like it when people give me information and they tell me how accurate they believe it is instead of me assuming everything they tell me is to be taken as accurate.
Italics mean the word wasn't in the original text. Using the link above, look in verse 2 where it has the word "and" italicized. That means the Amplified translators put that word in to make it's easy to read and have grammatical structure, but I shouldn't read that word and run off on a tangent in my interpretation and believe things like "that verse says 'and' not 'or' " or something like that. Plus, one of the criticisms against the NIV was that they "took out some verses" and stuff. That's because the NIV looked at oldest known copies of the manuscripts and decided anything not in the oldest copies wasn't in the original and, therefore, shouldn't be in their translation. The AMP translators decided to include those in their translation, just let us know using italics that they weren't in the oldest manuscripts. I like having that information.
Then there's how they elaborate the words in case there's ambiguity. Look at John 3:16 in John 3 and see how they translate the word "pisteuo" ("believeth" in KJV). They're making sure we don't see the word "believe" and think it means how we often use the phrase today like, "I believe it might rain by noon." They say "believes and trusts".