IT depends on how you want to play. First of all are you aiming for solo pve or group pve? I'd recommend having a build for both.
For solo pve I think assassin as a main will be your best bet. You will have strong burst damage to kill mobs quickly, invis to help you navigate through difficult situations, and cooldowns to help you deal with adds. For group pve you might want to look into riftstalker or bard. Riftstalker will allow you to tank for groups, and bard offfers good support buffing and healing.