The idea groups I have never used are only Espionage and Naval.
Contrary to what others are saying, I like maritime, mainly because it gives you that nice boost to base naval tradition, light ship combat (I love my light ships), and especially the increase to naval force limits. Of course, as mentioned, that last perk has been somewhat nerfed by the trade company dynamics...since you get additional naval force limits from assigning provinces to them.
While I like espionage in theory, there are simply too many better ideas to choose from. Although it is now made more attractive by support rebels causus belli, until there is a covert action that amounts to "assassinate monarch or heir" or "increase dynastic influence" I will be passing on it. I will settle firmly on espionage being presently the least useful idea group.
Naval, I imagine can be useful and I actually intend to try it on my next run as a naval power - particularly England. Grab Innovative, Naval, and Maritime on England and their navies would be completely unstoppable. Alternatively, if you are playing Italy, Venice, Genoa, Ottomans, etc, they benefit strongly from the galley boosts - in which case number of heavy ships DOES NOT matter by comparison to the galleys. The only reason everyone is saying that ship counts matter more is because they have yet to be faced with a naval equivalent skirmish to fighting Prussia on land. You know, when you have nearly twice the army size but it gets almost completely wiped out while their army took minimal losses? That one. Experience that and you may find naval to be more to your liking. Many of you are just so used to manning deathfleets and going against comparably weaker opponents, but do play a multiplayer match when someone is actually playing GB, Netherlands, or Spain with great naval efficiency for a contrast of perspective.
Probably my personal favorite idea group is Humanist. I almost always go protestant or reformed, so it guarantees I survive the reformation without religious turmoil. It is great for expansion (especially with the new autonomy system) in reducing nationalism and increasing cultural acceptance. Decreased unrest, combined with high levels of tolerance, you will never have revolt problems again. The ability to quickly assimilate conquered and annexed provinces makes this highly valuable for mainland expansionists. Finally, the decrease to idea cost is also wonderful, and for that reason I usually grab this idea group first.
Religious synergies well with humanist ideas, giving you that extra 20-25 base prestige (depending on whether you have innovative ideas or not) and that stability cost modifier are excellent. While it could be argued that it is better to take one or the other (in which case I usually lean toward humanist) they do work together well in that humanist keeps the peace while religious allows you to convert to your religion and culture more efficiently.
Economic and administrative have their place, and to be honest I think I'd lean more to economic than trade since the wealth of nations expansion.
Plutocratic gives that wonderful tech bonus, plus all of those trade bonuses and increase to morale.
Altogether, I think every tree is redeeming in some way for your style of play - except espionage (unless combined with a nation with espionage bonuses, like Ireland).