What is the opposite of ad hominem

In business-speak one often hears that a message benefits from a "Halo-Effect" because the messenger has a good reputation for one reason or another.

Appeal to False Authority (or argumentum ad verecundiam) is a logical fallacy that means more or less exactly what you said: "something is given more credence than it deserves because of who's saying it." But then the real opposite of any logical fallacy would really be a correct argument, so in that sense @Vladtn is right.

Also, ad hominem does have another form, where an attempt is made to reinforce an opinion based on the positive image of the person who shares it. Practically all celebrity endorsements in advertising use this fallacy. When you see Einstein brushing his teeth with Crelm toothpaste, you want to buy Crelm, right?

ad rem would be the opposite of ad hominem, as what is pertinent, to the point, regarding the topic of discussion rather than to the interlocutor.

Edit: if your question is about unjustified (celebrity) testimonial, I don't believe there is a simple expression for that (although there should be!), but to describe your example and if you don't mind leaning a bit on the formal side you could maybe go for non-authoritative endorsement. Other ideas?

The argumentum ad hominem fallacy refers to a rhetorical move which discredits an argument by characterizing the person making it.

By “opposite” you seem to mean a rhetorical move which gives an argument more credence than is deserved, also by characterizing the person making it. That is the fallacy of appeal to authority or argumentum ad verecundiam.

But if by “opposite” you were to mean a move which discredits an argument in a valid manner instead of a fallacious manner, Vladtn has your term: argument to the point or argumentum ad rem.