One song

Joey - Caught in a Mosh

Bush - Only

There's dozens you could name, really.
 
from the joey era i would go for an unpopular choice and go for medusa. I think that songs kicks the old backside. And from the john era i would go for what doesnt die i think that song is just the ticket.
 
Since I started this thread, I might as well throw my two cents in.

For Joey, I'd say Caught in a Mosh, and for for John, I'd probably say Inside Out. Although there are other song's of John's that I like more, I think that one is fairly representative of what they play.

Now, that being said the thread title was "One song", so in choosing between the two, I'd pick "Inside Out", because that's more representative of who Anthrax is now.
 
Joey era - Among The Living
Bush era - Inside Out
Overall I'd pick Inside Out as my one song. Like nafnikufesin said Inside Out isn't exactly my favorite Anthrax song but it seems like it would be a good pick to get the attention of an Anthrax virgin.