Best level 6 unit.

you can also consider them as creatures that join your army))

Hmm..., diplomacy - is it an assessment with an emphasis on it? Interesting... But what is the point of such a comparison? I think diplomacy is generally useless; you might as well consider Wizardry or even the number of level 1 creatures capable of negating all the advantages of level 6 creatures.

let's say you have one slot available, who would you take? ))
so I would rather take strong infantry than weaker flying units))......

You say you don't like flying units... But when defending a Castle, as in your example, Ifrit Sultans would be useful... Just imagine, the walls are breached, and I don't think I need to elaborate further... And what about such a trick as Armageddon? - isn't that an argument in favor of Ifrits?

... and everyone has different tactics))

It would be a sin to disagree with that...