This is a misconception I always see online, sometimes by Arabic Teachers and native speakers. While yes, there are a lot of variants, the choice should be made obvious by the teachers (who I blame). Anyone who wants to learn Arabic, should learn Modern Standard Arabic (or Quranic, they're the same) for these reasons (non-exhaustive): 1- It is the most understood variant, as it is the lingua franca between all Arabic speaking peoples (and beyond). 2- It is similar to most other variants (Basically it's pretty near the vernacular variants of Peninsular Arabic, and not that far from the Levantine, Egyptian, and North African ones) 3- And MOST IMPORTANTLY : Unlike the vernacular variants, it is a written language with codified rules, clear grammar, and a vast lake of vocabulary resources.
Slightly related rant:
The most annoying idea I see spread all over is the comparison of Arabic and its variants to Latin and romance languages. Which is as misguided as a bent arrow fired from the hip. Latin is dead while MSA is live and kicking, being used daily by millions if not billions of people. Because of the Quranic staticity, Arabic is a pretty much "Frozen in time" language with little evolution. (A phrase written 1600 years ago might still be understood today, unlike in English or French for example)