Hejaz
Historically, the Hejaz has always seen itself as separate from the rest of Saudi Arabia. The Hejaz is the most populated region in Saudi Arabia; 35% of all Saudis live there. Hejazi Arabic is the most widely spoken dialect in the region. Saudi Hejazis are of ethnically diverse origins.
Arabic
Within the non-peninsula varieties, the largest difference is between the non-Egyptian North African dialects (especially Moroccan Arabic) and the others. Moroccan Arabic in particular is hardly comprehensible to Arabic speakers east of Libya (although the converse is not true, in part due to the popularity of Egyptian films and other media).