Warrior is pretty easy, given great armor, some interesting weapons, and good ranged and melee abilities that flow well in many groups.
Guardian was fun as well, but suffers a bit more at ranged, but rocks at melee, at least to me. Great survivability though between armor and various skills.
Mesmer is fun, but as a light armor class, lacks the survivability of the other two. Skills are fun though, although, group zerging xp isn't as effective with this class compared to others.
Engineer is fun, but wow, limited weapon choice means you spent a lot of time with one of your kits instead.
Honestly? iId so go warrior, then once you get the hang of the game or get that class to 80, try out one of the others.