I would go with Midnight Reaper and Isareth.
Reaper has relevant typing with zombie in case you're running Gravecrawlers and it replaces itself once it dies. I like that it encourages aggressive play while having your opponent make decisions about whether it's worth it to let you ping yourself and draw a card off trades or eating attackers with a bigger creature. I'm all for more interaction.
While Liliana has more interesting play to her once she flips, Isareth is also a solid card. It was a house during M19 prerelease, especially if you had any way to fill up your graveyard with choice cards to reanimate. Looters work well with her, and in the worst case scenario she'll trade with another card before she hits the graveyard. I'd personally lean towards Isareth since I already have two Lily walkers in my cube, but you can't really go wrong with either one here.