Choose a fun one, like archer or thaumaturge for the main class and a tank as a second class. So you'll get into the Duty Finder dungeons instantly (but the ones mandatory for the main class, sure) and collect plenty of update 2.1 bonuses… Also it's good because the biggest Tank skill needed is to lead people on dungeons since they are the ones going forward. You'll get to see the other first with the main class and then you can replay them with no waiting time as a tank.

At least this is what I wish i had done.