I recommend any mage follower, specifically, ice and fire mages. 
Here's why: There's nothing worse than a kiting dragon, it flies around and then gets you with its fire/ice breath then it lands 100 metres away and you have to run over and by the time you get to it, it flies off to kite you some more.

Mages have a nice distance attack that they'll use on a flying dragon and then they can attack from a distance when it lands.

Archers also have a distance attack but dragons have weaknesses to fire and ice... So go for mages.

There are a bunch of them throughout the game. Try Marcurio, J'zargo, Brelyna Maryon or Onmund.

For a full list of followers and where and how to get them, go here

[Skyrim Followers UESP](http://www.uesp.net/wiki/Skyrim:Followers)