I don't tend to get annoyed by warmongering nations who declare war for no reason because I've just accepted that this is a common thing in Civ 5 and I always prepare myself for invasion, regardless how friendly my neighbour seems to be. I don't really enjoy Domination victories (unless I'm provoked into war) and if I see a Civ getting too big for it's boots I'll send in a smack down. I prefer cultural or science victories. So I get royally pissed off by the likes of Venice, Austria, Brazil and Indonesia who all seem to be dominant due to their various cultural benefits.
For example, Austria joins EVERY city state in marriage and the map becomes a sea of Red and therefore her cultural/diplomatic output is always really high as a result. Venice buys City states and also has a lot of cultural influence due to trade routes. Brazil has lots of cultural benefits because of their Carnivals and stupid amount of great people and Indonesia gets all this happiness from island cities. I also hate Civs that send a basillion missionaries into your territory such as India. I've declared war on Civs just to get rid of their damn religious nut jobs trying to convert my people to their heathen God(s)