Horde mode for sure.
While Firefight and Warzone are fun, repetition got old really really fast. And annoying.
Call of Duty Zombies can be better, as I'm not AS focused on the maps as much as I am surviving. Making the replayability a little better, but not as much. As it does get infuriating.
Never played MGS, because it's the worst thing since Caboose.
Horde's got better since the Gears of War 2 days. Fortifications, Weapons, Mech Suits, Turrets, Sentry's, and a ton of other stuff to build up for yourself while having a fun, and the maps don't even matter ever most of the time as you get to choose which one you want to play.
Horde mode is better, and Beast Mode is insane.
I like Halo, but Gears will always have the better play.