for fast leveling PvE:
1. 1st job - Soul Hunter - slow to begin but becomes the best after lvl 50 = 2nd job Defiler (for fastest xp at high levels), 2nd job Dominator (for AFK grinding). Also good in PvP
2. 1st job - Templar - fast to begin, slows a bit solo, but gets AoE pts easy = 2nd job Radiant, also important for PvP
for good PvP:
1. 1st job - Hunter = 2nd job Ranger - easiest class for PvP... longest range and very simplistic control
2. 1st job - Battlemagician = 2nd job Elementalist - very powerful caster class with a lot of utility. In hands of good player class is amazing.
3. 1st job - Defender = 2nd job Commander - the best 1v1 class in the game. Low damage but high hp and numerous skills which inhibit players from attacking, ccing, or otherwise escaping you.