I went with Aela the Huntress. She is not invincible, but she favors bow and arrow so generally she's behind me, attacking from a ways away while I'm mixing it up with the enemies at melee range. Plus I liked her voice and my character is a werewolf too so it seemed like a good fit. She has died a couple of times (only ever by friendly fire when she wandered right in front of my sword), but I save frequently enough that it wasn't a big deal to just reload. I had her in Elven or Glass light armor most of the time (Forsworn armor if I felt like having a little eye candy around ), and it wasn't a problem keeping her in something as long as it has better stats than her default outfit. You can also give her one really good arrow like a Daedric arrow and she will make infinite copies of it as long as you keep her equipped with her default bow.
I actually used Aela for almost my entire playtime until just switching her for Serana last night after starting the Dawnguard DLC.
I've encounted a major bug after Dawnguard was installed that the game no longer recognizes Aela as my wife, she now stays at the Companions home, and someone from the Companion quest line has come back to life.
Also beware that even if he/she is sleeping, you cannot vampire feed from your spouse. They will instantly detect and aggro you. Anyone else seems to be fine. Haven't tried to use the vampire seduction but I believe they may be immune to it.