A cube based around a generic Set Skeleton, but with the best versions of each type of card available (Wind Drake commonly appears in sets, but Vexing Gull is an upgrade for the same cost and p/t.)

3:2:1:1 Rarity Distribution
Commons: 18 of each color, 10 artifacts, 6 lands.
Uncommons: 14 of each color, 1 creature & 1 noncreature for each color pair, 5 artifacts, 5 lands.
Rares: 7 of each color, 1 per color pair, 5 artifacts, 5 lands.
Mythics: 2 for each color, 1 for each enemy color pair, 1 colorless.

Planeswalkers purposely excluded