The most egregious tampering, IMHO, was Micah 6:8.
Original NWT:
He has told you, O earthling man, what is good. And what is Jehovah asking back from you but to exercise justice and to love kindness and to be modest in walking with your God?
Revised NWT:
He has told you, O man, what is good.
And what is Jehovah requiring of you?
Only to exercise justice, to cherish loyalty,And to walk in modesty with your God!
Just absolutely butchered. What a beautiful thought, to "love kindness". And now it is rendered as "cherish loyalty" which as you can imagine, they have already used repeatedly to underline their core message of "OBEY US or die".
Even the exclamation point at the end irks me. "I'm sure the original Hebrew writer would have put an exclamation point in there, if he would have been writing a thousand years in the future when they actually, you know, existed."