Seven mana is a lot to ask in a world of fast aggro, but Ugin, Eye of the Storms is worth every single one
Creature - Cleric Snake Color tag templates(None) Sivriss, Nightmare Speaker , Sacrifice another creature or an artifact: For each opponent, you mill a card, then return that card from your graveyard to your hand unless that player pays 3 life
Thats a grand total of 6 mana for a 1/1, and you cant even go infinite since its a once-per-turn deal
Those should both be in this list, but I don't own a copy of either yet
If you do, create an X/X black Aetherborn creature token, where X is the amount of paid this way
There are ways to remove counters, move counters, get value for having counters, and spend counters