Marcurio is the best by far, he's an Imperial Destruction Mage. He rarely gets in my way, has a high level of Mage-skills, so much so that he often Zaps the most powerful foes to Oblivion AND he has a funny sense of humor. per uesp wiki Skyrim, "Marcurio is the only hireling available in the base game who will use Destruction magic against enemies. He will also use magical staves." I've traveled with a few (Lydia, Faendal, Stenvar, J'zargo, Cicero, Janessa, Onmund maybe some others) Marcurio with Barbas has been the best for defeating the strongest. Barbas gets in the way a lot, but he's awesome at distracting and weakening foes for my final kill and he kills those pesky skeletons in one swoop.