Unfortunately asking ppl what the 'best' class is in a MMO is sort of like asking a group of 3rd graders what their favorite flavour of starburst. You will never get the same answer, none the less 1 solid answer.
Its all opinionated, players who prefer a melee/fighter class will say Bagi or Azure Knight every time. As per players who like casters/healers will pick Incar, Segnale, or Summoner. So on and so forth!
Since it is beta, and if you aren't too sure what your particular preference is in reguards to casters, melee, ranged. Maybe you should take this time during beta to try out each character you think you may like to level 15-20, see what you think, then from there look for some in depth posts in reguards positives and negatives to the class you enjoyed most. Since we all know each class has them.
That being said, I myself would say that a Incar or a Dagger spec Hunter would be the 'best' (if best is technically my favorite, or what I have seen do really good in game).
And why, well simply a mage has the option in 2moons to be the regular weak behind the scenes damage dealer as per most typical mage types in most mmo's. Aswell as has the ability to be built strong, with heavy damage, aswell as defence to be able to very sufficiently stand his/her own in solo, pvp, and group combat. And finally the best part of a well built mage is his AOE potential. I think I speak for every member of my guild when I say dealing 8k damage (each) over 3-5 seconds to 5-10 enemies at once when we are guild grinding in Norak or Castor is a complete lifesaver! And I stand behind that statement 100%, especially since we have not once needed a segnale with us because our AOE DPS is so mass!
Then the why for Hunter, simply put, DPS! You may look at the Hunter's spec tree for melee/dagger combat and see that it only consists of approx 7 skills. Well honestly that is more then enough, now unfortunately they dont have much past level 50-60, which hopefully will change in official release, but the mass Damage per second they deal out with the few skills they have, makes them very deadly in pvp, and solo grinding.
Unfortunately not having my hunter very high level at the moment I cant comment on the AOE abilities (if any) for a Dagger Hunter, but imo the DPS alone as long as stat built properly to hold minimal damage while DPS'ing down the other enemies. Should be ok. I can validate these statements when I get my hunter higher lvl.
Well theres my 2 cents!