In my opinion, I'd probably list the following:
Most annoying Ritual spells:
1. Utterdark - if you're willing to cast this, you're one of the few nations unaffected by global darkness or a 90% income drop (probably undead). Unfortunately, everyone else is going to be most unhappy with your devastation of their armies and tax base, which is going to make you public enemy number one.
2. Burden of Time - Kills the elderly, which means a dangerous amount of mages for many nations. Undead don't care, and the very young aren't quite as affected. Also gradually kills your population, which means that if you have a gold-based economy (read: aren't Ermor or Lemuria), you're going to have issues if this stays up for too long.
3. Astral Corruption - Horror show, but definitely not horrorshow. If an opponent has this up, kiss your non-blood mages goodbye if you cast or forge anything, which is a bad thing in the magic-heavy late-game. And if you try and fail to dispel it? Horror-marked. Eugh...
4. Strands of Arcane Power - As if creeping dominion wasn't bad enough, now it spontaneously mind-hunts your mages. The only bright side is that if the mage that cast this ritual gets feeble-minded due to lash-back from a failed mind hunt, the ritual ends. The down side is that any mage that casts this is S7 at a minimum, and will not be easy to defeat in a Mind Hunt duel.
5. Wrath of God/Vengeful Water - See Arcane Power, only with less "targeted destruction" and more "death from above/below." Annoying, but unlike Strands, a bit less dangerous.
6. Illwinter/Second Sun - bad enough for most nations, but if you're actually a nation of the opposite temperature from whichever one was just cast, these rituals mean trouble. Expensive (especially Illwinter, at no less than 120 Blood Slaves!), slow, and ineffective, but supremely annoying.
Other things to watch out for are gem generators - these aren't dangerous in themselves, but they're powerful gateway spells if they're placed in the hands of a powerful nation. I'd probably rank them from most to least dangerous as:
1. Arcane Nexus - "What's mine is mine, and what's yours is mine." Any spells you cast turn into gems for this one's owner.
2. Stellar Focus/Well of Misery/Blood Vortex - "I win" buttons abound in these Astral, Death, and Blood, as long as they have the spells/forges for it. Very dangerous, high potential for a runaway game. Well of Misery is especially dangerous because it's *beneficial* to other nations, too - they might actually ignore it for a while if it's stripping away disease and ailments from them.
3. Earth Blood Deep Well - Beautiful forges in Earth especially paired with others. Potential to summon Kings of Elemental Earth.
4. Eternal Pyre - Some decent forges for Fire, and expect greater potential for evocation spam and siege magics. Potential to summon Kings of Elemental Fire.
5. Gale Gate - Queens of Elemental Air can jump across maps with high MOV and Flying, giving strategic flexibility if you're (somehow) an Air nation that doesn't have Flying. Decent evocation spam potential here, as in fire. Randomly spits out storms in enemy provinces, which is unfortunate since it makes you a bigger target.
6. Mother Oak - A couple good forges (Rejuvenation/Regeneration are both in this path), but that's about it.
7. Maelstrom - Water has mediocre forges (except for things like Frostbrand), but two of the three Queens of Elemental Water are only useful in the water. On the other hand in the water, they are most dangerous in their element...