We have absolutely no reason to believe Donald Trump over any/all of our top intelligence agencies. Trump has been proven a liar time and again, and cannot be trusted. He doesn't want to admit that there is yet again an embarrassing story about him. The intelligence agencies work to thouroughly and diligently sort through evidence to get to the truth and gather important information.
In any smear campaign, you have to ask yourself who tipped the cops and other oversight groups. If the incident really happened (as in the agency verifies it as a public act), then the celebrity or politician in question should expect to answer some forthright questions. That's the role of United States national security agencies anyway -- to verify evidence presented them by law enforcement and the judiciary.
Yes, National Security Agencies can be trusted. The information against Trump that they've gathered was collected over time. Many people were involved in the gathering and analysis of this damaging information. Of course Trump is going to call it fake news. What other defense does he have? The louder he cries, the guiltier he appears.
No, members of the intelligence community have proven themselves untrustworthy. It came out late last year that at congressional briefings, intelligence officers have lied under oath, claiming they provided the "least untruthful response." That's an admission that national security agencies deliberately violate the public trust. Regardless of how one feels about Trump, there's no reason anymore to just take the word for intelligence agencies without some independently verifiable corroborating evidence.