Wayward Servant
Any creature you play is a ping to their life total since everything you play is a Zombie. While spending mana on creatures is okay you got plenty of ways to create multiple Zombies at once with tons of token generators like Gisa's Bidding, Liliana's Expertise, Stir the Sands, From Under the Floorboards, and Dark Salvation you will get great bang for your buck on this card.
Plague Belcher
Boy I love this Zombie. While Wayward Servant pings them when they come into play this bad boy pings them when they die. So no matter what -- coming in or going out -- as long as your playing Zombies they're losing life. Plus for 3 mana this 5/4 that can only be blocked by two or more gives you a distinct advantage in combat and something if they don't answer makes short work of their remaining life total. Only 4 unblocked swings. Take down their numbers so they cannot 2-for-1 you and then go in unblocked.
The only downside is you need to -2/-2 something but as long as you have a
Tattered Mummy
in play that becomes to your advantage too. Kill it off, they lose 2 life, bring it back with Relentless Dead. That or put them on something like the 2/3 Lord of the Accursed. BTW if your Zombies have any Anthem effects in play giving them +1/+1 they wouldn't actually die to the -2/-2 counters. The other pro move later in a match would be to activate
Shambling Vent
for 3 mana to a 2/3 then cast Plague Belcher putting the -2/-2 counters on the Vent. They'll disappear and nobody has to die for the cause.
Relentless Dead
Finally it gets it time to shine. This Zombie loves low CMC Zombie friends. Cards like Dread Wanderer can come back on it's own but Zombies like
Fan Bearer
and
Festering Mummy
are exactly what it's looking to bring back into play. Ensures you always have a Zombie in play by bringing one back for X=1 or X=2 or X=3 none of your Zombies cost that much putting them all within range for recursion.
Dread Wanderer
Nice 2/1 one drop.
Festering Mummy
Fan Bearer
What this deck needs are more cheap one drops to keep the pressure on them.
Cryptbreaker
loves em for the draws and so does Relentless Dead.
Tattered Mummy
This often overlooked and underappreciated little guy is actually really tactical in how you play him. A scenario i imagined while brewing up this deck would be to attack in with
Tattered Mummy
have them actually chump block it with a 1/1 but not kill it. They think they are saving themselves from damage but then giving it -2/-1 with Liliana which then kills it and pings them for 2 life loss plus if the Belcher is in play another 1 life loss for it leaving/dying on top of that. It's like getting to cast a free Lightning Bolt every time. Then if that doesn't end the match you can always bring it back later with Relentless Dead dying to get more burn damage usage out of it.
Diregraf Colossus
One of the cards you almost unanimously see played in Standard Zombies decks but I don't consider it a must. I include it more for the ability to generate tokens faster than other spells.
Lord of the Accused (AKH)
Anthem effect for Zombies + gives Menace. What more could you ask for on a creature?