Originally posted by SolaScriptura
BDAG is the best lexicon.
I occasionally look at Liddell and Scott, but they focus on classical Greek - much older than NT Greek. It is occasionally helpful to see how a word was used hundreds of years before the NT writers use it, but for why this is really indeterminative (and sometimes irrelevant) see Carson's "Exegetical Fallacies" 2nd ed. pp. 35-37.
I would also recommend:
Moulton & Geden's "A Concordance to the Greek Testament." 6th ed. Ed by I. Howard Marshall.
This is hands down the BEST concordance to the Greek NT.
With BDAG and Moulton & Geden, you are ready for some serious NT studies.