My current follower, Marcurio, is my favorite. He's a very powerful mage (now he's a tank mage) which compliments my character who's a full warrior. Apart from that, he's the most "realistic" follower since he gives out advice at certain location (e.g. Watch out for traps / Look behind you for enemies, etc) and also reacts to the environment (e.g. The cold is unbearable..). Only tiny thing bothering me is whenever I have to trade something with him, he always complain with that same line: " I am an apprentice mage, not a pack mule.." (something like that) But so far, he's my absolute favorite.
I settled on Marcurio after Lydia got killed early on in my first playthru. He is the perfect foil for my gruff Nord Warrior both in playstyle and personality. By the time I'd finished he had every staff in the game and was really powerful.
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The thing about followers is that early in the game they are very week. You are best going thru a few meat shields until you get past lvl 25 when you should be choosing a follower to take you to the end of the game. In my second playthru as a Mage I went thru Argis, J'zargo, Onmund and dopey Vorstag, who didn't have the happiest of ends...
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I have since settled on Cosnach the Drunk, who has even funnier lines than Marcurio. He is kitted out in dual enchanted ebony, has a unique enchanted sword and is very much a beast
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he can basically go face to face to face with a dragon while I hit it with spells. Those two folllowers have made the game 20% better for me.