Word of Crota
It's situational. For crowd control, Fatebringer. For bosses and ultras, Word.
I have 3 of each and anyone who votes for WoC has never used the Fatebringer. The FB only does slightly (very slightly) less damage but Firefly deals much damage to surrounding enemies. When you can kill 5+ thrall per shot on Crota HM with the Fatebringer or one at a time with the Word.... there's no contest. Even against high level knights it only does 10 less damage but blows them up when they die, doing damage to surrounding enemies. FATEBRINGER is superior by far.
Fatebringer still wins this debate. WoC is only better when you need void damage
Fatebringer. WoC only good for VoG
WoC all the way