kinda hard to say... depends what you want them for...
Cicero is pretty good for a stealth follower
Aela is pretty good if you want an archer
Serana is pretty good as just a "jack of all trades" kinda follower..
Frea is pretty good for a melee based follower
Erandur is probably the best mage follower you can get...
however, you can have one NPC follower, and one pet follower at the same timje if i recall, so if you want a follower I suggest you grab one of the ones mentioned above, and then go grab one of the following pet followers as well:
Armored Troll if you side with the Dawnguard
one of the Dawnguards Husky's
Meeko (the dog)
a Riekling Warrior (not a popular pet follower, but a good one in my experience)
Steadfast Dwarven Sphere
or s Steadfast Dwarven Spider...
or do what I do, and just go solo...
Depends on playstyle. You want a mage, an archer, or a melée tank? Hate when someone gets in your way, or is that no big deal for you? Want someone who talks a lot or keep quiet? Want someone you can marry?
There have been a lot of threads about this, you could always check http://www.gamesas.com/topic/1472513-best-vanilla-follower/ out, it was the most recent before yours, I believe.
My favorites are probably
Mage - Marcurio
Warriors - Vilkas, Erik the Slayer, Vorstag (who also favors the bow)
Archer/dual-wielder - Jenassa
Spellsword - Teldryn Sero
Lydia.
She's a great pack mule. I mean a good distraction companion.
Then there's Serana, who literally can't die.
Right now I have an orc male that I picked up in an orc camp South of Markarth. Can't recall his name right off hand, he is the one that tells you there are many types of bravery and that you have a grim look. I have not had one problem with him. Jordis has been pretty good too. Too many problems with Lydia not being able to jump down rocks with me and she is always literally right in my face.
http://i.imgur.com/IkjHHns.jpg
Best of all, he's portable. You can even count on him to help you escape from prison! Or even if you find yourself trapped in another realm! Nor does he yap and yap about mortal inconveniences.
This. Dremora is all you need.
As for actual followers, I find that they all have very little difference between one another to actually claim one better than the other.
Best Followers would have to be Serana, Frea, and Teldryn Sero. Sero does max at level 60 so that's something to keep an eye on.
I feel like I'm the only one who doesn't use an escort/companions. They just always seem to get "in the way" for me.
When I did the quest with the dog, he was probably the best companion I had. He can't die and killed the average bandit/spider alone while I was searching for loot.
Assuming you are playing on a PC, you can get the construction set and look at their stats. Using this process, I chose Vorstaag when I needed a tank for my mage, and Jenassa when I wanted one to complement my stealth character. She dual wields and uses a bow.
The stats for Lydia, as I recall, are the worst of the bunch, which makes sense since she joins you for free.
Darkbrother Hood Assassin Initates ARE the best followers in the game.
-Essential (immortal)
-Max level cap is 100 (most followers cap at 25, 30, 40, and 50)
-Zero morality (witnessing crimes won't incur bounty)
-Master sneak (won't be a hindrance in stealth)
-Master one handed (specializes in daggers, swords, axes, and maces)
-Master marksman (can use the bow top tier, but you have to provide the arrows)
-Master light armor (doesn't actually matter, you cannot change their armor and they can't die anyways)
Unless you want a mage follower, they are the best follower as far as melees and archers go.
Jzhargo technically would be the best late game if you want a mage since he doesn't have a level cap, although at early levels, there are arguably better followers.