Answer:
Young drivers must remember to always turn off cell phones before getting behind the wheel.
Explanation:
This has the strongest emphasis for me, because 'always' is a very harsh and completely unambiguous word in this context.
The other split infinitives, "to not text", "to fully focus", and "to completely ignore", do not have the same effect, and sound odd and unnatural, whereas "to always turn off" sounds much more definite.
Additionally, "to fully focus" contains alliteration, which trivialises the serious matter being discussed (safe driving), as it 'poeticises' the statement somewhat.
