I learned the under the leg, torreando and over/under all in the same week. Though I've added a few (hundred) other passes over the years, the over/under's been my go to 'til this day, 18 years later.
Two goals when teaching a white belt a guard pass (or any technique, for that matter):
1. Give them something that actually works, that can be used effectively throughout belt levels (with modifications as time goes).
2. Give them something that helps teach and reinforce principles that are applicable beyond the technique you've shown.
With that in mind, I would suggest double underhook pass, over-under (single underhook) or knee cut pass. All three passes work at all levels if done correctly. Both passes require the development of fundamental BJJ skills/ideas - controlling distance, controlling space, upperbody/lowerbody control when passing, and elbow awareness (not flared, not extended).
They also happen to be passes that can be "forced". It is easy to enter these passes from most guard positions. This is useful for someone who doesn't know a lot of techniques and wants to get live reps of a guard pass.