Shogun 2 just has a perfect mix of having the best AI in the series so far, great visuals, good optimisation, sleek, yet functional UI, a ton of features and improvements over the previous games and really well working mechanics, both on the strategic and tactical maps.
If they had picked Rome's setting for that game, it would be pretty much perfect. But, as it it, it's just Japan.
shogun 2 ai is one of worst i ever seen.
they camp like fuck.
they use archers to kill your archers that are IN cover, and they succeed.
what is worse that they can snipe over walls and kill those who are not in cover.
their general is suicidal.
many more problems that i dont really have time to write down here.
imo, medieval 2 had better ai, even tho it was a little dumb at sieges. but its not like they went full retard everytime. sometimes they were quite competetent.
note: this is the list of things that bugged me when i played unpatched vanilla, it is highly probable that CA fixed those issues.
EDIT: they also cheat like fuck and have no upkeep. when you see an ai player with two provinces, and three full stacks of troops running around, you know something is wrong.