Where to first?

SM2 got me hooked on 40k and I'm burning through the 1st 4 books now using this reading order. My question is what line should I go down first? I'm thinking Fulgrim or thousand sons. Thanks in advance!

SM2 got me hooked on 40k and I'm burning through the 1st 4 books now using this reading order. My question is what line should I go down first? I'm thinking Fulgrim or thousand sons. Thanks in advance!