In EU4 for everything you need to do, there is almost always at least one other way of achieving that goal. Income is the most obvious example of this. You can make money with trade, tax, production, razing, looting, forcing money out of vassals and forcing other nations to pay you through direct payments or war reparations and finally by dev'ing gold mines (which is a kind of pseudo production income). There are probably other ways that I'm forgetting.
Given this dynamic of substitution, one way that you can look at idea groups is to examine what they give you that is difficult to replicate in some other way.
For example, Religious is somewhat mediocre because the main draw for a normal SP game is the CB. In today's game, there are rando claims from estates, much more developed and extensive mission trees that you can directly or indirectly leverage, etc. In short, there are ways to work around the lack of CB.
Ultimately, this is why certain idea groups are much better than others -- they give you something valuable that is difficult to replicate.
The best example that I can give you is Offensive Ideas -- every player needs to siege things down, and Offensive ideas is the only set that grants you directly Siege Ability. Certainly, you can gain more Siege ability in other ways, such as improved professionalism. However, having Offensive's Siege ability and a bonus from Professionalism is better still. In fact, the only thing in an idea set that helps you with Sieges is the extra siege pip in Aristo (which is one reason I think it is under-rated).
Admin gives you that gov cap and, of course, some CCR -- but, there is some (a lot possibly) dead weight in their. Mercs are great early, but decrease in value as the game progresses.
This, of course, can change as the mechanics of the game change.
Diplomatic Ideas is perhaps the most valuable idea set in the game because it grants you two diplomats, which are much more powerful than they used to be, given the new favor system. Diplo gives you two diplomats, improved relations, improved reputation, an extra relation and -20% warscore cost. This whole idea group in today's game is essentially fat-free -- not one idea here is dead weight.
The flip side of this is Trade ideas. Once upon a time, the extra merchants that you got from trade were great, because it was hard to have merchants. Now, even a relatively land-locked power like Russia can have tons and tons of merchants without trade because of trade companies. You might think trade is good for other reasons, such as the policies, but it is objectively not as good as it used to be.
If you view ideas through this lens substitutability lens, then your tiers are something like:
S -- Diplo, Humanist, Offensive
A -- Admin, Exploration*
B -- Quantity, Influence, Religious*
C-- Aristo, Expansion*, Defensive
D -- Econ*, Trade, Quality, Espionage, Innovative
F -- Naval, Maritime,