As Olin Moyle's Ghost said, the question really is a fallacious argument - after all, 1] the WT comes up with a unique concept of Jesus Christ's kingdom 2] Other viewpoints of the kingdom are derided as worthless, even if other faiths publish a hope based on the kingdom of God 3] the WT takes a literal and important view of Matt 24.14, making it a key point of their campaign - so who else is preaching WT doctrines ? Nobody..of course !
If one allows the discussion to descend onto the fallacious circular reasoning that the WT employs, then you are onto a loser..the only hope is to keep the discussion on a higher level, if that is possible with your J W acquaintances.