It's probably better for you to use the API that serves your purposes than the one that seems, by some arbitrary evaluation, the most popular.
Your answer will also depend on what you mean by usage frequency.
My guess is that AutoLisp has the most users and is the most frequently used.
Other than that, your answer will depend on what you want to do and if you can acquire the skills to do it.
You'll need to understand how AutoCAD works.
You'll need to be proficient using the language used by the API
You'll need to learn the particulars of the API
Regards,
// Called Kerry or kdub in my other life.
Everything will work just as you expect it to, unless your expectations are incorrect. ~ kdub
Sometimes the question is more important than the answer. ~ kdub
NZST UTC+12 : class keyThumper<T> : Lazy<T>; another Swamper