As explained here , companians usually don't die.
Additionally, it seems that every companions does have the same stats.
They also can use very different weapons and armory, you just have to give them a better weapon and they'll use it.
gaming.stackexchange.com/users/328/Sven
I have tried a few of them, and I really don't see the big difference - they might as well be the same.
A good tip would be too take all their stuff.
Replace it with a bow and some better armor, this will make them not run in to the battle and get hit by accident when in 'wounded' mode.
Then they will die very rarely, but they will not make a good Tank this way.
It is quite useful if you use spells or a bow yourself.
gaming.stackexchange.com/users/8416/Allan Kimmer Jensen
Marcurio is amazing, I have played with him over 24 hours and hasn't died yet, I could even lay down and let him kill dragons.
Marcurio is a male Imperial that lives in Riften.
He can be found in the Bee and Barb and can be hired as a companion for 500 .
He is a great follower using mainly destruction spells and can kill most things with ease.
However beware having him near friendly NPC's because he mainly uses chainlighting and this has been known to jump to them and cause a fight/bounty.
Source: http://elderscrolls.wikia.com/wiki/Marcurio
gaming.stackexchange.com/users/17077/Gabriel
Barbas cannot die and tanks like a boss.
Once you get the quest to retrieve an axe for his master, use him as your companion and never complete the quest.
gaming.stackexchange.com/users/14659/Joe
The best companions to have are the members of the Companions.
The reason is that all regular companions in the game are at least 6 levels under your own level, but not the Companions.
You can get them to follow you if you are their leader, after destroying Kordlaks beast spirit.
And remember, the Companions are not only good followers, they are also trainers.
Farkas is the companion with the most dmg in the game, if you get him the right weapon.
Trains for heavy armor.
Aela is a ranged who wont stand in you way all the time and can sneak very well.
Trains for Archery.
Vilkas also has great damage, but is not as tanky as Farkas is.
Trains for Two-Handed weapons.
gaming.stackexchange.com/users/15144/Skyrimaddict
Complete all the Dark Brotherhood quests and get Cicero.
When you get a choice to kill him or keep alive, keep him alive for a companion .
He's a super strong assassin.
gaming.stackexchange.com/users/24315/Tyler Landa
To find the right companions use your judgement.
If you want a tanky companion look for a beefy guy with a shield and an axe.
Want a mage?
Look for a scrawny guy in a robe.
There is a companion in every main city (they are mercenaries and cost 500g).
Try looking in the Inns for these companions.
Another thing to note is that their base equipment is pretty bad, so if you can afford it or make it give them some upgrades.
They will use the best base damage/ armor in their inventories, so make sure that the weapon/ armor you want them to use is the best one they have.
gaming.stackexchange.com/users/14291/Skizzlefrits
I went with Mjoll the Lioness, as she's a quest NPC she can't actually die.
She just goes into a downed state and then gets back up after the right.
Pretty much making her invincible, which is a shame really as sometimes it'd be nice if she shutup.
gaming.stackexchange.com/users/5357/David Yell
As far as I'm concerned, at some point out in front of Windhelm some of the cat traders should be camping out.
I believe one gives you a side quest where you must retrieve his "Moon Amulet" from some bandits.
I believe the quest givers name is Kharjo.
He becomes available as a follower shortly afterwards and has a tendency to kill everything that comes your way.
I stood and watched as he slayed and elder dragon in front of my face, all ebony gear.
gaming.stackexchange.com/users/23654/TwistedChaos
From what I remember companions' stats are based on your level when you acquire them .
That's why Lydia is often considered the weakest; because she is usually acquired very early game when you are low level.
I never had that problem with her, because I wait until I am much higher level before getting her
and then she can tank alongside me against almost any foe.
It does help to give them buffed (enchanted) armor, of course!
gaming.stackexchange.com/users/130069/ZealousHypocrites
The main determinant of a companion's power level is their actual level.
Before patch 1.6, followers would get a level when they first spawned, based on the Dragonborn's level, and then stay there.
This meant that the later you met a follower, the stronger they were.
This was fixed, and they now level with the Dragonborn, up to a cap.
This cap varies depending on the companion.
The Elder Scrolls Wiki section on Followers' Leveling says that: Followers will match the current level of one's character, up to a certain level cap that varies from follower to follower.
The vast majority of followers cap at level thirty.
Roggi Knot-Beard and Sven notably cap at level twenty.
All recruitable members of The Companions who are not part of the Circle, as well as Adelaisa Vendicci, cap at level twenty-five.
Mjoll the Lioness, Illia, and all hireable Mercenaries (including Erik the Slayer) cap at level forty.
Also, in the Dawnguard add-on, Beleval and Agmaer cap at twenty-five.
Erandur, the members of the Circle (Aela, Farkas, and Vilkas), and all Housecarls cap at level 50.
In Dawnguard, Serana caps at level 50.
However, there are some companions who don't have a cap.
If you are a higher-level dragonborn, then clearly these are the strongest followers: The followers who, before patch 1.9, would level with the Dragonborn to 81, after the patch have no level cap and will continue to level with the player.
They include J'zargo and the Dark Brotherhood Initiate , and with Dawnguard and Dragonborn, the followers Celann, Ingjard, and Durak, along with Frea , are added, respectively, with no level cap.
gaming.stackexchange.com/users/108003/DCShannon
I've had a few companions who have died.   What are the best companions from a health perspective? And from a damage perspective?
Strong companions in Skyrim
gaming.stackexchange.com
<the-elder-scrolls-5-skyrim>
gaming.stackexchange.com/questions/36199
gaming.stackexchange.com/users/12035/Coomie
Companions' stats are based on your level when you acquire them. Most cap at 30. Marcurio, Barbas and Cicero are difficult to kill.  Companions have the same stats based on the player’s level, with some exceptions.
It is suggested that companion’s level is linked with the player’s level. Certain companions and their stats are being described.