I switch between Aranea and Onmund. Onmund is a wicked battle mage. He has skills in Destruction, Heavy Armor, and One Hand. I put enchanted ebony on him and a paralyze enchanted sword for when his magicka is low. I give Aranea two staffs and she will dual wield them when her magician is low. Just don't give her a chain lightning staff anymore. O.o
I'm using Aranea and I'm curious why shouldn't I give her the lightning staff..? lol
Anyways, to answer OP's question, I think any companion is good enough if his/her skill complements yours. So for mine, I'm playing a 2-handed, heavy armor warrior, (no magic at all, just sometimes for restoration) so I'm using Aranea as my follower cuz she's fully magic-based and she can summon atronachs, so when I'm doing close-combat, she can strike the enemy from range with frost spikes or something. : )