Organized them into categories based on my opinion
Eh, up to you really.
The Great Betrayal series
Three Ogre short stories (Golfgag's Revenge)
Claws of Chaos (Gav Thorpe)
Rise of Nagash Trilogy
You should probably read these, at some point.
Priest of Mars series
DO NOT SKIP THESE!!
The Talon of Horus
The Return of Nagash
Malus Darkblade series
Rebirth (Can I read this without reading the first trilogy?)
Pariah (Will there be a folllow up to this?)
For the former, no. Likely not. The Circle of Fire trilogy will feature carry-on stories from the Tome of Fire, so it's worth reading them first. And for the latter, yes. Penitent by Dan Abnett should have been out by now, but something is keeping it. It'll be out eventually, I hope.