Each Magic card has a rarity and, as a general rule, rarer cards are usually better
Granted, you have to fill your graveyard with different permanents first before you can summon the Spirit God The Ancient One, but that's very easy to achieve if you're playing Dimir colors, anyway
After activating these filters, I find that the most expensive single for pre-order as of late July is Tezzeret, Cruel Captain
It can protect valuable cards from black discard, and can combine with library shuffling effects to improve your draw (such as Thawing Glaciers, Flooded Strand, or Ramosian Sergeant)
This is my favorite new artwork in the entire set, so the fact that it appears on a very popular card guarantees an appearance on my list