I was asked to subtitle a student movie. I don't care much about which subtitle format to use - I'd probably use vim for typing them, and still it is interesting which formats are most common and what are the benefits for using them.
SBS - Australia's Special Broadcasting Service has an extensive and international reputation for quality subtitles. from http://www.sbs.com.au/aboutus/faqs/index/id/120/h/Subtitles
SBS subtitles the majority of its non-English language programs in-house. The distinctive colour, font and size of subtitles were adopted after extensive research. For the majority of our programs, which are broadcast in colour it was found that yellow letters with thin black borders provide the maximum degree of contrast. For black and white material we use white subtitles with thin black borders.
Madman is an Australian distributor (originally of anime) their subs match SBS:
Most of the time Madman uses Arial, size 36. The yellow is straight-up no frills yellow: R=255, G=255, B=0.
A similar match is these settings (obviously size depends on your video aspect ratio).
Font: Verdana, Regular
Primary Color: Yellow
Outline Width: 1
Shadow Depth: 1
Border Style: Outline + Shadow