Use type faces that match your brand. Use two typestyles. Differentiate with type sizes and use of bold text. Make it readable.
Sometimes, three or more type styles make sense. Use a serif and a non serif for headings and text, but use another for impact for important words or messages.
Or use color to focus the eye.
Do not use all caps and do not underline. This is very old style, from back in the day when people used typewriters that did not allow for multiple typefaces and color.
Click here for the expanded version: 5 Typography Tips for Every Presenter.