You guessed it. The answer is MEGADETH. Slayer Clinches the number 2 slot and Metallica slightly edges out Anthrax for 3 because it has more unique words.
Does this mean that lyrically Megadeth has the smartest lyrics, the best phrasing, rhyming, scansion?
No, its all a matter of taste, it just means they have the largest vocabulary of the big 4. Its a fun factoid.
Largest number of words counting only one occurrence each: