Monarch can take Thanos in a straight up fight with no prep time. The feats he displayed in Countdown Arena and Countdown show the scope of his power.
Blackheart has the capacity foe immense power. At his peak, I might be tempted to say that he can defat Mordru who as a Lord of Chaos wields massive mystical power. If Blackheart is at full power, then team 1. If Black isn't at full power, then Team 2.