I've been experimenting with this food since it was added to the game and based on my experimentation for Unholy DK's, 125 stat food will always be better for AoE/Cleave fights due to how multistrike affects our Necrotic Plague damage. For single targets fights like Zakuun Felmouth is the food to use for sure.
"Multistrike causes your abilities to have a chance to hit/heal a second and a third time, each time dealing 30% of the initial damage/heal." Due to this, for heave AoE/Cleave fights buffing Multistrike will be more beneficial due to NP ticking more often and it can be spread around without end doing lots of damage.
Here's what I've been using..Assault: 125 stat, due to lots of adds that NP can spread to.
Reaver: Felmouth, purely ST.
Kormrok: Either works but 125 stat food is better for hands, even if you're running defile, MS makes it tick more.
Kilrogg: 125 stat and spread NP through all adds.
Council: 125 stat, have NP ticking on all 3 of them.
Gorefiend: I've always used 125 stat food here due to how much damage NP does in the burn phase, it's insane.
Socrethar: 125 stat, spread NP around all targets.
Iskar: 125 for sure, great for NP on adds.
Zakuun: Felmouth for sure, just like Iron Reaver.
Xhul'horac: 125 stat food is far better, so many adds.
Tyrant: 125 food and Felmouth and very close until the last burn phase, then NP ticks a lot harder and there's where the dmg really matters, so 125 stat food here.
Manny: 125 stat food, spread NP to imps and infernals and let that bad boy tick.
Archimonde: 125 stat food, this is very important for P3 when infernals spawn, you wanna spread NP to all of them and kill them ASAP. It's also great for domcallers and spirits in P1 and adds in P2.
Conclusion: Fights with adds, use your stat food. Single target, Felmouth is a safe option.
"For fights that are purely single-target, you should always eat Felmouth Frenzy, especially if there is movement involved. The proc of Felmouth Frenzy is RPPM, so it is not affected by movement (it will still proc as often in a movement-heavy encounter), but it does not scale up with additional targets, as stat-based food does. For this reason, this is the food of choice for Fel-Lord Zakuun.
In other situations, you should always eat food that gives you Multistrike." -Skullflower, Ex-Midwinter DK and therorycrafter.