Best level 6 unit.

If you start comparing units based on the benefit they can bring in battle, then the Nagas clearly have the most advantages. Other units certainly have their own strengths, but 30 damage, 16 attack, and a retaliatory strike – in my opinion, this is incomparable to paralysis, blindness, and poison, which trigger once in a blue moon.
However, I agree that Death Knights, Magic Elementals, Champions, and Efreet Sultans deserve consideration. The last three are good, each useful in their own way, but the topic is still called "Best unit of level 6".