Which civ do you find the most interesting and/or fascinating?

I’d say Mongols, both in AoE IV and previous games.

In AoE2 they had -imho- the most fun campaign with a superb start recruiting different tribes and then the China scenario and those unforgettable words: “…China, the most advanced empire in the world!”.

Also the Mangudai are (and were, in AoE2) like my dream unit.

Historically wise they changed the world, for good or worse. Some historians say that the human population at the era took a big hit 'cos how they conquered other places.