In my experience, the base game AI factions that do best tend to either be militaristic, or expansionist (or both.) The expansionist ones build cities, while the aggressive ones capture them - either way, the other AIs expand too slowly and can't compete.
That said, by late midgame, on difficulties where the AI doesn't get overwhelming bonuses, these initially successful AIs start stalling out because they can't manage unrest, allowing the other AIs to start catching up (or, more usually, let me start taking them over.)
The AI construction/research priorities don't seem to mesh well with a "tall not wide" strategy.
As far as Mod factions, I find that CoS's Frost Giants can sometimes do well without one of the above traits as the way their unit design works (single initially expensive but powerful units become more relatively efficient to build as tech increases) tends to compensate for some of the problems with the way the AI prioritizes. That's only if they luck into building a decent fortress or two, though.
That said, I'm probably going to start trying some of the recently developed AI mods to see if those improve things...
