1-51: Class Quests & Hunting Logs. (Kill things and do FATEs for missing exp in-between levels)
^Can be done in < 4hours (1 gaming session)

15-50:
The slow time...
#1 Queue up for Party Finder of Duty Finder and run dungeons. The best you can!
#2 Run FATEs while waiting for dungeons and parties.
#3 Hunting logs wherever possible
#4 Class quests
^In order of importance and experience p/hour

Some people will recommend leves.
Yes, they are fast, but if you're a serious gamer interested in ALL aspects of the game.
Save them for crafting.

FOR YOUR FIRST JOB, JUST RUN STORY QUESTS AND OCCASIONAL FATES FOR MISSING LEVELS. Experience the story and get carried to 50!