Best level 6 unit.

Here is a comparative overview of all level 6 units (upgraded). It is unclear what criteria are used to evaluate the best unit in this topic, so judge for yourselves.
Champions
Attack 16; Defense 16; Damage 20-25; Health 100; Speed 9; Cost 1200. Abilities: increased damage with charge.
Efreet Sultans
Attack 16; Defense 14; Damage 16-24; Health 90; Speed 13!; Cost 1100. Abilities: flying, fire aura.
War Unicorns
Attack 15; Defense 14; Damage 18-22; Health 110; Speed 9; Cost 950!. Abilities: blinding, aura.
Cyclops
Attack 17; Defense 13; Damage 16-20; Health 70; Speed 8; Cost 1100. Abilities: ranged attack, siege weapon.
Death Knights
Attack 18!; Defense 18!; Damage 15-30; Health 120!; Speed 9; Cost 1500. Abilities: double damage, curse.
Naga Queens
Attack 16; Defense 13; Damage 30!; Health 110; Speed 7; Cost 1600. Abilities: retaliates.
Monarch Wyverns
Attack 14; Defense 14; Damage 18-22; Health 70; Speed 11; Cost 1100. Abilities: flying, poison.
Scorpioids
Attack 16; Defense 14; Damage 14-20; Health 80; Speed 11; Cost 1050. Abilities: flying, immobilizes.
Magic Elementals
Attack 15; Defense 13; Damage 15-25; Health 80; Speed 9; Cost 800. Abilities: retaliates, attacks all around, magic immunity.