Wizard: High burst Damage, Weak Armor, Good mobility. Great utility (you can raid effectively, you can port).
Necromancer: Damage over time (DoT) kiting class. Low armor but higher survivability than wizard due to crowd control and shield skills. Unlike wizard which has to use 2-3 projectile skills, Necromancers are entirely mouseover or AoE skills, making them easier to use if your aim isn't the best.
Bloodmage: Easily the most complex and cool class on the server at the moment. It can DoT, heal (although it requires smarter play to heal effectively compared to other healer classes), it can do good damage solo, it can use a great mechanic called BloodBond to turn on support mode. Out of mana to heal/fight? Transfuse takes health for mana, letting you have reserve burst if you can kite and heal effectively.
All three are classes that are fun and very competitive at level 60. Bloodmage is underplayed currently because many of our players choose less complex classes, so if you want to be unique, I would go BM. If you do not want to walk while you raid, go wizard, and if you want to be able to go to Hell and also survive longer than any other caster spec without having to aim a single goddam skill go Necromancer.
If I had to guess which was strongest in pvp, I would say from strongest to weakest Necro - Bloodmage(skilled) - Wizard, but all 3 are strong classes right now.