I've played all the factions in every time period and I would have to say that I've come to prefer the 1580 time period, especially for its difficulty.

For an idea of just how aggressive the AI can be, look here:

https://forums.totalwar.org/vb/showthread.php?t=112231



Hardly a single season went by where I wasn't faced with at least one invasion and sometimes several

The main thing about the 1580 date is that, due to the extensive infrastructure granted to your opponents, it's difficult to turtle (although I suppose you could if you were really determined) and you must play aggressive and move decisively. While I've certainly had my share of memorable campaigns playing all the other starting dates, it just seems like in 1580, your entire fate can rest on the outcome of a single battle far more often, and that knife-edge is probably what attracts me to it more.