I would have to agree with the above post. J'zargo is, correct me if I am wrong, the only follower that levels up with your character, so he is very good to keep around if you are in need of a follower. However, if you are a lower level, J'zargo is hard to get. You must unlock him by doing some college of winterhold quests and one of his quests which might prove a challenge for those who are low levels. I currently have Mercurio, and he does wonder for me at the moment.
In my opinion, if you are a mage, you probably don't want a follower because you will have spells that do damage in an AOE (Area of Effect) which might kill your follower. If you use a bow more than a sword, a melee follower is what you want, with heavy armor and a nice sword and sheild or two handed weapon. If you favor the sword/mace/axe, then a mage/archer would better suite you.
The top followers in my opinion would be these:
Melee (Non-Ranged)
Anyone from the Companions
Melee (Ranged)
Faendal
Derkeethus
Mage
J'zargo
Mercurio
(Keep in mind, the melee characters are really only as good as the weapons and armor that you suite them with)