Justinian often finds a way of being very problematic...He expands well with Imp, techs decently, has a strong UU considering AI love of mounted units, and also tends to develop allies w/religion. I've seen him be dominate in many games and pull some dirty tricks I never would have thought the AI capable of...
Two examples:
1. I was prepping for a canon/draft war and had just finished the Globe with the help of some chops and slavery (the city had no hammer tiles whatsoever). With the globe built and nationhood adopted (also switched to caste as it seemed preferable over slavery once the globe had finished), I'm ready to go into draftathon mode to get units for my canons... Then, on the EXACT turn I want to start drafting, I get a notification that Justinian spy-sabotaged the granary in my draft city!
Just incredible. I was really kicking myself for leaving slavery...
2. He manages to foul up yet another canon war. I had just taken Steel from Liberalism and used a GM to upgrade my catas/trebs. I'm ahead of the pack in terms of research, with a monopoly on gunpowder still intact, and already moving the troops in place to backstab Justinian. The situation looks very good; his big SOD is in a close border city, conveniently on the way to his capitol, and only two tiles deep in his culture. Around four turns before DOW Justinian manages to research Military Tradition. Eh no big deal. However he must have realized something was up, because only two turns later he proceeded to steal gunpowder from me! In the few turns it took to move the stack to the SOD-containing border city, he upgraded at least 12+ cataphracts to cuirs and immediately flanked the absolute sh!t out of my canons. Then he bribes his buddy Hannibal into the war and continues to spam cuirs...He pretty much pulled the
only possible move that could have saved him.
I definitely spend much more time scrutinizing war situations as a result of my encounters with him.
Gotta vote for Ragnar as well, I've seen him use galleys to unload an army in my back territory even when we're on the same continent+sharing borders. Nasty.