What's the difference between BDAG and the others, Thayer's especially?
By the time Thayer's lexicon came to be published in the late 1800s, discoveries of new papyri had been made in Egypt, making Thayer’s life work unfortunately nearly instantly obsolete. As a result, scholars realized that a lot of the words that were only found in the Koine Greek of the NT were actually part of the common language of the day – prior to that, since they could not find any parallel between NT Koine and the vernacular of the day, they thought that God had used a special Greek language to reveal Himself, hence, the term the “Holy Spirit Greek.” Long story short, over the next 100 years Bauer, Arndt, Danker and Gingrich, compiled the most up-to-date lexicon. In Greek studies, BDAG is just the ultimate. Nothing compares to it.
So a good suggestion is, unless you are a 1st Greek student, chuck out your Strong concordance and lexicon, Vine and Thayer’s, and rely nearly almost on BDAG. If you are involved in academics, no one will value your work if you rely on Thayer and others like that.
I suggest you check Wallace, pp. 14ff, and Decker’s Koine Greek Reader, pp. 245 ff, for more info.