This is kind of an apples and oranges thing, to some degree...
Peace and empire building is, bar none, the cornerstone of the game - but a peaceful building gets you eaten alive by a military nutjob who essentially is only as long-term effective as the people he/she can conquer.
Diplomacy can be a viable substitute for military against the AI - but against players, it's pretty much useless, unless you include cash bribes in diplomacy.
Civics? They're very, very potent - but bottom line, they're probably the least integral of the lot. Sure State property is all kinds of spiffy, and free market can boost your research X,Y, and Z... But mainly they just bolster what you've already got going for you. It's the icing, nothing more.
I'm choosing option 5 five with "not a very good question to ask, really" in mind. None of these are impotent, and if they were, only situationally, some are integral. Military and peaceful building are core gameplay, and there's nothing even close to impotent about diplomacy and civics. This is kind of like asking, "what's more impotent - the engine, the wheels, or the steering wheel in your car?"