Who are the best coders to follow on GitHub?

I think this heavily depends on the language you're interested in.

1.) Generally speaking, you may want to have a look at the following search result on GitHub - listing all users with more than 1.000 followers and sorted descending by follower count: https://github.com/search?o=desc.... 
Assuming that people that already have a lot of followers are worth to follow, this should give you a good starting point.

2.) You can also limit this search to a specific programming language. So, for example, to get all users with more than 1.000 followers, mostly active* in PHP** and sorted descending by follower count, search as follows: https://github.com/search?l=PHP&....
Assuming that people that already have a lot of followers and are mostly active* in a language you use are even more worth to follow for you, this should give you an "even better" (suited for you) starting point.

Paul Irish (Google)
https://github.com/paulirish

Kit Cambridge (Js/Ruby whizz kid)
https://github.com/kitcambridge

Peter Michaux (JavaScript guru)
https://github.com/petermichaux

Harry Roberts (CSS whizz kid)
https://github.com/csswizardry

Steven Levithan (Regex legend)
https://github.com/slevithan

Addy Osmani (Google)
https://github.com/addyosmani

Jeremy Ashkenas (Backbone.js/CoffeeScript/Underscore.js)
https://github.com/jashkenas

John-David Dalton (FuseJs/Microsoft/Js guru)
https://github.com/jdalton

Christian Johansen (BusterJS)
https://github.com/cjohansen

Brian Cavalier (When.js/CujoJs)
https://github.com/briancavalier

John Hann (CurlJs/CujoJs)
https://github.com/unscriptable

James Burke (RequireJS)
https://github.com/jrburke