If you have a special interest in a particular part of the Arab world, or if you have friends or family from a certain area, go ahead and learn that dialect. If you want to learn Colloquial Arabic but have no real leaning towards one variety or another, Egyptian or Levantine Arabic would be the best choice. This isn’t necessarily based on linguistic reasons; it’s more because of media exposure. Egyptian Arabic is the most widely understood dialect, thanks to the well-established music, TV, and film industry there. Egyptian media is popular enough that no matter where you go in the Arab world, you’ll keep hearing Egyptian Arabic on TV and the radio. So if you learn Egyptian Arabic, people all over the Arab world will be able to understand you easily.