Mortivore would be pretty good, as long as it has a stable power and toughness before setting off the chain, it will be extremely buffed at the end of the chain. The only problem would be that a lot of graveyard return will only weaken it, particularly something like Grave Betrayal, pulling your opponents dead creatures onto your side will mean that the Mortivore won't get the power and toughness boost from a creature being in a graveyard while still giving it the consecutive -1/-1 from those creatures dying. Overall meaning that if you aren't careful you could easily kill one of your potential late game powerhouse/win-cons.
With mikaeus, the Unhallowed and a walking Ballista you can ping your opponent for 1 damage over and over till ues dead. Mikaeus gives all creatures undying so when you remove the 1 counter from Ballista to deal a damage he enters the grave as a creature with no counters so it's an infinite loop if opponents cant immediately respond to 8t
Phyrexian Plaguelordseems decent too.