Fonts for subtitles must be easy to read quickly on a TV, movie or computer screen. For this reason, it must be what's known as a sans serif font. A serif is the little protrusion on letters that accents the basic letter. Sans serif fonts do not have serifs, which makes them clearer on screens. Though virtually any sans serif font would work in subtitles, there are favorites.
Popular Fonts and Outlines
The most popular fonts for subtitles are Univers 45, Antique Olive and Tiresias. More important than the specific sans serif font, though, is the placement of both a soft shadow and a very thin outline behind the type. This keeps the subtitles clear no matter what is "behind" them on the screen. When something bright or white moves on screen, for instance, subtitles without a shadow and outline may disappear, or become too difficult to read. Another way around this is making the subtitles yellow, but the filmmaker may not want such a bright intrusion in her film.
Go With Your Favorite
As with any design element, typography is subjective. As long as you use sans serif with an outline and shadow, you are fine. Use a font that you like and that others can read. You may want to avoid the most universal fonts such as Arial and Helvetica for the very reason that they are seen so often.