No one class is really better than another class. They are all balanced to an extent, each have advantage and disadvantages, against certain foes, other classes, and quests/missions. The choice will be down to what style of play you enjoy.
For example,
Warrior = Hack and slash with a weapon of your choice.
Ele = Throwing fireballs and nuking until the cows come home.
Necro = Create armies of minions from dead friends or foes.
Mesmer = Make people hurt themselves if they want to hurt you.
Ranger = Stay away and damage them from a distance.
Monk = Look after team mates and smite down those you don't like.
blah, blah, blah
The best thing to do is try each of them out in pre-searing. Even then your selection may change later as you play. I started with Warrior, Mesmer, Necro and Ele. After a being in post-searing for some time, I deleted and recreated. Now my 4 characters are Warrior, Mesmer, Ranger and Monk. The Mesmer and Monk are the ones I enjoy playing most.
