Of the best fonts to use for subtitles, all of them are sans serif. A serif is a small line at the end of a character, and sans means "without," so you want fonts that are simple with no protrusions.
Three of the most popular fonts for subtitles are Unvers 45, Antique Olive and Tiresias. All three fonts are thin, sans serif fonts and should be used in their standard form -- that is, without bold or italics.
The color of your font should almost always be white. Colors like yellow and red will be highly visible, but bright colors like these tend to bleed and distract from the other material on screen. White is the simplest font color.
Because your font should be white, whenever your subtitles are placed over a white or bright surface, it won't be as visible as possible. Use a stroke -- a thin, black outline -- around your text to separate it from the background. Adding a faint drop shadow will also help to create some distance between the subtitles and the background.