chaos before conflag, because first immolate takes time to be"detected on the mob" , so when you cast immolate you have to wait about .5s to register on teh mob before you are able to cast conflag, in my case it shows on the mob when im almost done casting chaos bolt.
In reality, when im raiding on my lock it looks like this:

curse, immolate,chaos bolt, conflag, spam inc. <- thats the starting rotation.
after that as long immolate is up, i will do conflag before chaos.
I heroics bosses die within seconds, so more casts squeezed in = more dps, thats also the reason why Im manually doing life tap before boss pull.