Base loot tier is C on whatever * you fight:
7* C = 7 Loot Tier
7* B = 8 Loot Tier
7* A = 9 Loot Tier
7* A+ = 10 Loot Tier
Titans: Grades, Loot Tiers, and You
Please see the link above for the more definitive answer. 
Base loot tier is C on whatever * you fight:
7* C = 7 Loot Tier
7* B = 8 Loot Tier
7* A = 9 Loot Tier
7* A+ = 10 Loot Tier
Titans: Grades, Loot Tiers, and You
Please see the link above for the more definitive answer.