I wouldn’t say there is an optimal font for subtitles, but there are certain aspects of a font that would make it suitable for subtitles: Subtitles are meant for being read fast, so you need a font that favors easy word recognizion. This would be a neutral font like Helvetica or Times New Roman. Times new roman I find especially good at the job, because it’s not too fat, doesn’t look horrible when bold, and most important; it uses serifs.