Well Taros just got revised to 6E (dropped in price to boot), and involves Elysians, Marines, and Tau.
DKoK was in all 3 Vraks books, and that was just nasty (Marines, GK, Chaos, and Krieg); the characters have been updated for 6E, and DKoK got their big update in Fall of Orpheus (which is 6E as well with the Minotaurs).
Mymeara was nice, introduced some Cadian armour IG along the Wolves and a pretty cool Eldar Corsair concept army.
HH-Betrayal is by far the best book I've got from FW, but is only Marines (big shock
) so any Xenos players will be irked to say the least. Of course, there's also the Primarchs and Marine Superheavies.....
TL/DR answer - I'm copping out, but go with what can get the most people having fun with