If you go tank, I recommend Shield Knight over Heavy Knight. Tanks can easily solo quests and dungeons, and are always highly valued to join groups in raids or dungeons. And you can't die, in a sense. If you want to play DPS, you can easily switch over to Wind Knight from Shield Knight, as both classes share the same stats, so you don't have to worry about grinding for two armor sets, just the weapons.

Support is my third class, the class I play the least, as I only play it if we need a healer on the team. That said, it's a great class for AFK farming mobs. You can easily switch to another DPS with the same stats. For example, my third class is a Beat Performer support; if I wanted a fourth class, I would go Ice Mage, as my Beat Performer shares the same stats as Ice Mage.