1998 is the latest.
No matter what you see on Ebay or hear from your friends, SDNers and/or other dental students, 1998 (released in 2003) is the latest NBDE Part 1 exam available.
You might have dental students jotting down actual exam questions immediately after a particular exam and compiled them into a word document, but these are not actual released NBDE Part 1 questions/exams.
For example, I took the NBDE Part 1 on July 15, 2003. Immediately after the exam, several classmates of mine typed up anywhere from 30-40 questions that they recall from the exam into a word document. These are unofficial exam questions from the July 2003 exam. Most of these questions are only questions without any multiple choice answer choices, they're all from what people have remembered. Could be useful, but not reliable. [NOTE: I do not have or access to the above mentioned document, do not ask me if I can get you a copy!!!]
I see and hear people selling 2001, 2002 and 2003 NBDE Part 1 exams and claiming them that these are the actual released exams.....all I wanted to do is laugh! DO NOT FALL FOR THESE false advertisements!!!
Study hard and good luck on the exam.