Here are a few that resonate with me (note that these are from a Christian/Bible perspective; of course, to the extent the Bible is a book of fiction, then that should be enough to discredit all so-called Christian religions, including the WTS):
#1 - False Prophecy. In the Bible, Jesus predicted false prophets during the last days. The WTS has predicted "The End" numerous times (1914, 1918, 1925, "within a generation of 1914"). And each time has been a complete and utter failure. Of course, just like a kid caught with his hand in the cookie jar, the WTS makes excuses, even torturing the meaning of the word "prophet" beyond recognition. When the WTS does this, it reminds me of former U.S. President Bill Clinton who famously testified that "it depends on what the definition of the word 'is' is."
#2 - Going beyond what is written. In the Bible, Paul commands not to go beyond what is written. The WTS does this all the time. For example, it has invented rules about sexual practices between marriage mates, banned organ transplants, and forbidden the celebration of the anniversary of one's birth. None of these things are touched on in the Bible, but the WTS has taken it upon itself to invent rules (much like the Catholic church and other denominations of Christendom).
#3 - Slandering God. The WTS teaches that "imminently" God will kill every man, woman, and child that does not have the "mark for survival" described in Ezekiel 9. According to the WTS, the "mark" includes being baptized as a Witness. Thus, the WTS teaches that God will "imminently" destroy billions of people--many of whom have never even read a Bible, much less a WT. The scriptural backing for this doctrine is tenuous at best.