When i did one of the Daedric quests, you where to follow the dog Barbos. Once you had reached the area, he gave you the quest to take Barbos with you as you fetched an item. Then i got Barbos + my companion. And Barbos turned out to be immortal. And i don't mean he could be harmed and then after a while got up with full health. I mean he could take unlimited punishment. And he loved to trap me in corners. Once i got so furious so i started cutting him over and over. Probably like 10-15 slashes before he became hostile and i could see his healthbar. Turned out he never lost any HP no matter how much i hit him. So either he has 100 million hp or he has 100% damage reduction. Anyways i used him to do many quests as he was a great tank. But he always blew my hiding so eventually i just finished the quest.

I heard from a friend that Aranea is one of the best companions out there. You get here if you complete the Daedric quest of Azura. She's a mage to begin with so she will summon a Flame Archnoth (whatever the name is) by her own. So you can summon one too to make a pretty good army. Also with full Dragon Scale armor she becomes a good tank. She have only gotten beaten once and i was quick with my healing other spell so she was down for like 3 seconds. When i had Lydia who was in full Dragonbone armor she got beaten several times and once she even died (although i reloaded). Also giving Aranea staffs made her a nuking machine. 2 staffs of Fireball and she nuked at least 1 each second. Killed off entire rooms in a matter of seconds.