I have been using Janessa from the start. She has a nice mix of abilities, archery, 1 hand, sneak, light armour and block. She ALWAYS uses her ranged weapons first and then moves to engage. You can accidentally kill her, but it is rare and a reload fixes that. Worst I have done was in Rift, battling all the guards. I still haven't revisitted that battle again yet
She does have a few quirky behaviours though. Her default gear seems to be leather armour, a shield, leather gauntlets and leather boots, an iron sword, iron arrows and a long bow. However, she will always prefer a guards helmet over anything you give her, no matter how much better the new item may be. She tends to equip heavy gauntlets and heavy boots whenever given the chance and there is never any telling when she will swap necklaces/rings. Sometimes she chooses a plain silver ring over a ring of major X___X.
The hirelings are just like the enemies, their skill, characteristics and abilities stay relative to your own. I don't think there is actually a best hireling due to that. I think it boils down to preference, play style and which one supports you best.
Janessa is currently fully equipped in Glass that I have upgraded to Legendary/exquisite and she does very well in every encounter I have faced so far. She does particularly well in the aggro department, especially with dragons. Often I have a choice of staying ranged or melee. because her snealk is unequalled by any of the other hirelings (aside maybe the one from the dark brotherhood) you never have to worry about her getting you spotted.
Also, I kept the dog from the "A Daedras best friend" quest (I just didn't finish searching for the axe). He compliments you very well. Except he is also very annoying. He pushes you around, blocks your movements and many times makes it difficult to get out of corners. BUT, he is very powerfull and never goes into submission. He is quick to attack and draw aggro from an enemy leaving you free to deal with them as you like.
Oh, and howdy, I'm new

Sorry about the thread necro...