Which card makes more zombies?
As long as you have four mana Unhallowed immediately gives you two zombies regardless of the situation. How many does Ranks give you? It depends. That means dependency and dependency is bad.
With Ranks, if you have at least two other zombies in play then you get another one during your next upkeep. Having two zombies on the battlefield isn't exactly a monumental achievement, but it's more difficult than you might expect.
Your playing a zombie deck so you should have plenty of zombies right? Not necessarily. You need to save room for Doom Blades. Maybe some Grasp of Darkness and Distress. You could just cut all these good cards for wimpy zombies to help feed Ranks but that's a bad idea. Don't swap out generally powerful cards for generally weak ones.
Zombies die a lot too. Creatures attack and block. Shock, Oblivion Ring, Day of Judgment, etc. will wipe out your undead servants, especially if your opponent sees Ranks on the table. Assuming you do manage to keep two zombies on the battlefield, then during your next upkeep you get one zombie - half as many as Unhallowed and a turn slower.
If you play another zombie and if none of your zombies die then during your next upkeep you will make two more zombies meaning after meeting all these requirements and waiting two turns Ranks finally makes one more zombie than Unhallowed.
Once you get this far with Ranks things are looking up! Barring a combat calamity or board sweeper you're probably going to get even more zombies. If your opponent hasn't killed you by now or hasn't deployed an army of Titans, then you might win. Zombies will spew forth. Two, four, six, eight, the possibilities are endless! Beware of cards that say "If you are already winning then you win by more. If you are losing then you still lose".
While Endless Ranks of the Dead is tempting, Moan of the Unhallowed will provide more zombies a majority of the time. I imagine if you cast each card ten times it would go something like this (warning: made up stats follow): Casting Unhallowed ten times will give you 20 zombies plus 4 for flashing it back twice. Ranks will give you zero zombies half the the time, one zombie three times, two once and ten once. Much worse.
One more thing. You're playing Cemetery Reaper and Call to the Grave right? These cards require zombies to work at full strength so put in cards that make zombies (Unhallowed) instead of requiring more (Ranks).