This is really a question of style. Each profession will level up faster or slower, depending on the level or hunting location.
If I had to make a rank, it would probably be the mages (Druid or Sorc).
This is because they have area magic and can do a lot of experience solo or in parties. I would put Mage first because it is a damage vocation, so it will kill the fastest mobs. The Druid would be second but not far behind as it is highly sought after for hunting at all levels and thus can easily get hunts that give more exp.
In second place come the paladins. They also have a lot of area damage, however, because they hit physical/holy and don't have as high a ml as mages, the ease of killing mobs is no longer as high and the longer you take to kill, the less experience you will get. .
Lastly would be Knight. Because he doesn't have many spells in the area and hits with physical damage, it's not so easy for him to kill mobs with such speed and without going beyond one box at a time.
But all of this will depend on the friends who will play with you, the professions that already have your party and, of course, what style of gameplay you like most.