Sounds like an easy question. My guess would have been “optic yellow” (based on the color used since 1972 according to wikipedia).
This question was posed by cgpgrey on a twitter poll. The answer really depends. Professional tennis players use a standard ball color that is best seen on television sets.
For the recreational player, you can get almost any color – green, yellow, white, pink, etc.
What is the best color of tennis balls according to you?