Karazor is a fantastic writer, his Dread Chitin is the best fanfic I have ever read. He seems to prefer writing about Twilight and Rainbow Dash - characters which he captures perfectly. A Foul Light Shines is the only FiM/Wh40k crossover that's worth reading and Outside the Reaching Sky is already looking very promising. Arrow 18 is notable as the most realistic take on Human in Equestria. Thessalonica Legacy was the first fic I've read and I still consider it one of the best.