Nothing.
Let me state that another way. The best title you can put on any slide is no title whatsoever.
Why?
As John Medina has written, human brains are easy to distract. This makes a certain sense in the history of survival: being able to notice something new in your visual field could be the difference between finding dinner and being dinner.
But when it comes to slide design, putting audiences first means you need to drop out all those survivalist distractions, and focus on the central idea.
Remove anything that is not the single central idea.
Delete titles: they delay your eyes from focusing on the main content of the slide.
Here’s a few reasons why:
Your audience does not (necessarily) look where you would want them to at first glance of your slide. Remember, the audience has not seen your slide before. Things that you have stopped seeing because you’ve been tweaking your talk so much could jump out to the immediate attention of an audience member. Take a slide like this one:

The photo probably attracts attention first because it has a lot of color. Some might first notice the text on the image. And yes, some in your audience might look to the title first.
So if you’ve written the slide with the idea that people will linger on the title to get their bearings, then much of your audience’s attention is not where you think it is. This sets you up to lose your audience before you really get going.
How? The audience’s initial gazes are probably more a mapping survey: the audience scans the visual elements of the image to start applying meaning to them. This process starts a din of “noise” in the audience’s mind as they start to resolve your central idea. If this noise level gets too much, an individual might look away from the slide completely to lessen the almost painful frustration of too much to take in.
Busy slides are noisy slides in the minds of your audience.
Lessening how long noise lasts in your audience’s mind means more time for them to focus back on you, the speaker. Titles should be the first to go for a very simple reason:
You will say the title out loud. After all, a slide is a supplement to your words, so why add the visual distraction of the title if you’re going to tell the audience that anyway?

But why is it that deleting the title feels so odd?
If you are new to audiences first design, you tend to apply document design sensibilities to your slides. In text documents, titles are of vital importance: section headings, figure titles/captions, and chapter titles spark the audience’s attention after they’ve slogged through a long text passage. These sign-posts also quickly establish the structure of the document to ease reading flow. (Sub)-titles can act as easy bookmarks for when the reader leaves the document for a time. When you’re reading a book, think of how much easier it is to jump back in at a chapter start than mid-way down a page of straight text.
But as I’ve noted previously, talks are not manuscripts.
Changing to a new slide, especially one with a different layout than the preceding one, is a spark for the audience’s attention. So you do not need titles to call attention to a slide’s context.
The slide calls attention to itself.
When you cut the title, not only do you save the audience from the din of orienting themselves, you get back a whole bunch of space to play with. Now your picture can be as big as the slide will allow. Or if it’s a text slide, you can use an even larger font, making it even easier to read. The bigger the element, the more clear it is, and the more central that single idea is on the slide. You’ve got a slide that looks like a billboard.
So cut the titles from your slides.
Unless:
The title is the only thing on the slide.
If you’re giving a long talk, title-only-slides can act the same way they do in documents: as section markers. These text-only slides can provide a convenient pause to bring the audience’s attention back in for the next arc of your inspiring narrative.
But use audiences first design sensibility: The Title Card.
This is a slide that is nothing but title. Chances are the slide before and after a title card looks different. So the layout alone will attract audience attention.
Title cards are an example of treating text like a picture. Used sparingly, title cards are great attention grabbers.
In summary:
Drop titles if there are any other elements on the slide, and let your oral comments be the title.
If you’d like to use a title to introduce a new section, put only a title on a slide, and use the slides that follow to hold the images and text of that section.
Put audiences first: keep their minds captivated but do not overload them.
