My vote for the worst Watchtower term is:
Circumcision of the heart!
I can't believe that there are people who can not only say "circumcision of the heart" with a straight face in front of the congregation, but defend its 'meaning'.
Other runners up in my opinion:
Spiritual paradise
Spiritual food
This system of things
Faithful and Discreet Slave
Honorable mention (not in WT but used in meetings):
"extra portion of Holy Spirit for..." (Used all the time in prayers (I've heard CO's use this) as a request for 'extra' holy spirit for someone, like it was a slice of pie, and you only get one portion, but you can request more)