Genuine aliens as in outer space folk of interstellar origin probably still tops the list, to which we must add inter-dimensional beings, and, slightly related to the later, good old fashioned demons. There are many types of aliens, by the way, and they don't all get along with each other by some accounts. Aliens are held to be evil by many, as saviours of mankind by others, and of mixed intentions depending on which alien race we are talking about. You had to know that this wasn't going to be easy or simple.
A good deal, perhaps the majority, of this alien material is propounded in what some call the "alternate" media, the media that is real and true and honest, that tells it "like it is," as opposed to the "mainstream" media which is deceptive, and controlled by International Zionism and/or the New World Order, or just run and controlled by stupid, conventionally-minded eedjits. Never mind all those conventionally-minded eedjits, the so-called alternate media is very suspect itself, full of racists and anti-Semites, and demanding that its gospel be swallowed whole by the self-claimed independent thinkers just as much as middle America accepts the nightly network news. The rugged individualists are in fact soft and squishy groupies moving in lock-step with their self-appointed leaders.
Conventional wisdom is absolutely useless in thinking this through. Governments have a reputation for deception and cover-up, and institutional academia makes fun of any PhD faculty interest in such things as aliens, except, of course, as part of a study in popular culture. It is a dilemma, as it is way too easy to either accept uncritically the reality of alien contact, or to dismiss it all with humour and condescension.
And don't even think about claiming the middle ground--it is a killing zone of credibility as bad as either extreme.