Quick answer: choose Warrior if learning enemy targeting, Priest if learning party support, or a familiar damage class if playing with an organized group. The best beginner class is the one whose job remains clear after a mistake.
Warrior
A readable front-line role that teaches targeting and defensive timing.
Priest
Excellent in parties, but requires attention to allies and resource timing.
Damage classes
Rogue, Mage, Brawler, Ranger, and Bard become easier when another player already covers recovery and control.
What not to do
Do not select a class only because an old tier list labels it S-tier. Current event routes, party size, and unlock goals change what is actually easiest.