Ah, this is a good question. And unfortunately there is no right answer. In general the Sapphire 1 (or Sapphire I) seems to have appeared most often, but you'll find many different radios appeared in Herbie even in the same movie. It just wasn't a detail they were very careful with.
There are two great pages setup with information about the radios. First, Brian, setup this web page showing clips from all the classic movies:
Radios from Older Herbie Movies
Then for a pic of the radio from the latest Herbie Fully Loaded movie we can see this:
Which BTW I got from a great site that has detailed screen caps (screen shots, screen captures, whatever you want to call them) from all the movie DVDs. You can see all of that here: Herbie Movies This has been a great resource for checking on parts and colors to make sure my Herbie will be an accurate replica. BTW if you go to his site (lobousland) and click on Resto you can see his pictures of his Herbie restoration.
When looking through the movies for Herbie details, there's enough variations in Herbie between the movies you can kind of pick and choose what features you want yours to have. I'm trying mostly to stick with the way Herbie looked in the very first movie although may end up including a 200MPH speedometer as seen in Herbie Fully Loaded, simply because that sounds so cool.
BTW since we are talking about radios, I haven't dealt with this person before, but I've had multiple recommendations that if you are looking for a rebuilt VW radio, this is the place.