It would be useful to know what player pick rates are these days. In fact, this is probably one of the most useful applications of "pick rate", because rather than actual idea group strength, we're actually TRYING to measure perception rather than reality.
In the past, things like quality and economic were clearly overrated in SP context, while religious/humanist were middle of the pack in pick rate.
Overrated: Religious. People are too lazy to get CBs, I guess? But either way, trying to convert stuff is an expensive joke when you're conquering anything quickly.
Most government forms can't get a 0 DIP CB without picking religious or taking very small gains/war. As a result, most nations shave DIP cost where they can via influence ideas + rivalry mechanic, then tank the extra DIP price tag otherwise.
I've seen more than one very high level player joke that religious is a 2800 ADM cost CB, and that's not too far from the truth. Nevertheless, I've also see them pay it depending on their goals.
These days conversions have also started to tilt back into being possible to keep up with expansion to some extent unless you're doing very fast one tags. Not perfectly, but enough that the unrest reduction matters.
AE impact, siege ability, extra diplomat, advisor cost reduction, and passive corruption reduction are all strong to very strong.
The problem with espionage is that it doesn't scale well. It had a brief niche as a 6th+ pick to mitigate corruption in the terrcorr patches at least.
Advisor cost and corruption reduction are both ducat savings. As far as ducat savings go, they're pretty good, but by mid game the utility is starting to get smaller since you get closer and closer to running +5 advisers + army regardless.
AE impact often goes the same route. After early game, it becomes increasingly more practical to just all-in a religious/culture group region, rack up AE to the cap of 1000, and annex it directly or into subjects. Once you're doing this (and for some players, it's pretty early), AE impact loses a lot of its luster.
Finally, espionage offers extra DIP cost reduction, via more + faster claims. Again, this stops scaling well, because absolutism and -score cost modifiers allow more to be taken, but not for more claims to be made. To the point where if you don't have religious, even influence's finisher offers more savings, until imperialism removes much of this pressure regardless. Claim discount is further harmed by the fact that everyone and their dog will start slapping you with counter-espionage, limiting the + espionage modifier benefit and catching spies often enough to noticeably reduce claim rate...compared to just taking all the things at 0 DIP and similar AE reduction (idea group vs holy war reduction), in practice this isn't so great. It would be somewhat better if it allowed claims bordering claims like the discovery age ability, but I doubt that would push espionage above the top two groups for SP.
Overrated: Humanist. You can just deal with the unrest.
You can "just deal with" most things. However, if you don't have humanist, you will have to fight more rebels, and that implies moving your armies away from enemy borders/offensive war effort and back to kill them. In the mean time, they cost you money through occupation (provinces don't generate income for you) + devastation, and more money/manpower after you kill them via reinforcement cost. For any scenario where you avoid all of this due to humanist, the savings are substantial.
Humanist also saves a marginal amount of admin points via keeping unity high enough to not be penalized for stability, and prevents wrong religion modifiers in newly captured provinces even the instant you take them in a peace deal, so it is part of the recipe for turning captured land into productive land ASAP.
In addition to its improve relations modifier that policy stacks with diplomatic, it's not surprising to see it routinely listed top 4, and it probably deserves that lofty placement in SP. It packages ducat savings/generation like other groups offer with a few unique benefits and snowballs the power offered by provinces sooner than alternatives.
As for mine, for SP:
Influence (underrated): I feel like a lot of players put this off longer than necessary/beneficial, it's worse than diplomatic and administrative on average, but it's still the second best way to reduce price of adding cores to your land and stacks with the best one via policy.
Exploration/expansion (overrated): You can prestige purchase or steal maps, both pretty early in the game. Aside from that, dropping regular colonies is a very expensive endeavor for the amount of/delay in returns. Exploration does at least let you drop colonies to fabricate claims, even if you don't pay for them, but that's now pricier to reach/abandon than previously and you can usually manage this via poaching colonizers or opportunistic peace deals/alliances. The econ benefits of these (including expansion) don't hold up to what you get out of the better groups, even as those groups help you expand more too.
Military groups (overrated, in SP): Quantity is probably the best of this bunch, just because you can carry more fronts simultaneously and more easily manipulate AI perception of your nation. Also makes it easier to threat-check AI stacks while still carpet sieging it down/assaulting forts. But taking any of these, including quantity, before things that make your expansion better in SP is quite a price tag in terms of opportunity cost, all while more recent patches in EU 4's lifetime has reduced the opportunity cost of not picking military in SP (because you can general spam into professionalism, using government/privileges to discount doing so, and gain some significant military benefits/manpower with extra MIL points, which are usually higher value than MIL development in SP).
Note that in MP games with more than a couple players, running the usual 4 expansion speed/efficiency idea groups with no military ideas is a recipe for punishment. How good an idea group is depends on your assumptions and goals. The above assumes SP games with significant conquest (up to and including WC, but not only WC). I don't think naval is ever worthy of consideration in SP for example, but I can envision some scenarios where you'd lose if you don't take it in MP.
