Why not get economic and quantity both?
My choices for military ideas are usually Offense, Defense, and Quality. Unfortunately, that leaves little room for Quantity. Even worse for Republics since Plutocracy will be added in the mix.
Administrative is pretty much mandatory, Economic and Innovative too.
I usually pick Expansion a a diplomatic idea. Its not great (unless your expanding in Asia, and then its rediculously uber), but its probably the least bad and gets rid of a bunch of floating DIP points. It gets you some diplomatic relations/reputation which is nice, trade power which helps your trade income, and gives you a merchant and colonist. Its not the best but its not deadweight.
So, 7 ideas picked. Given, Expansion can be swapped out for other dip ideas if you need them, such as naval for naval powers and exploration for explorers.
The 8th could be any of the following -
Any DIP Idea: A lot of countries will need 2 dip ideas for whatever reason. One example would be Naval + Exploration.
Religious: I dont need to explain why this one is good, but imho some countries can go without it which is why I listed it here.
Aristocracy: Also provides manpower and a myriad of other bonuses. Enemy Core Creation seems useless but it actually is pretty useful in that it deters the AI from invading you (The AI Actually looks at how much it will cost to core your provinces before launching a war for territory). There are a number of decisions that provide permanent, decent economic bonuses. The only bad thing is that you MUST pick Aristocracy + Economic early, as Innovative idea group will disable many of those decisions. Also, do NOT pick Aristocracy if you are not getting Economic as well, but chances are if your getting aristocracy you are getting economic as well. The modifiers from the permanent decisions are -5% Stability cost, +5% Global Tax, and +5% Production Efficiency. Not great, but not terrible either.
Quantity: If you really dont like Aristocracy, and Religious is not for you, in this case id pick Quantity. Again, my complaint with Quantity is that the bonuses are too small, +33% Land Force Limit is pitiful, and -10% Cost/-10% maintenance will barely put a dent in your bottom line. It really fails in letting you field 'moar' troops. Only the manpower bonus is good but army buildings are still by far the most effective way to fix any manpower 'problems'.