Liquor isnt a natural liquid. Also if you do it like this, everyone will KNOW what they´re drinking which means very soon there will be some changes made, like getting new water from whatever nearby source that can be found.
How is it unnatural?
So how about this for Aquam:
Instant Squall or One of Our Navies is Missing (ReAq 40)
Range: Sight, Duration: momentary, Target: part
A continuous area of water of up to 20 square kilometers and up to 5 paces deep (finesse roll for complex shapes) flashes into steam. Anything floating on that water will be thrown about by the steam then fall to the bottom of the affected volume as the steam fails to support its weight. If that doesn't smash all the ships then the waters rushing in from the sides and the impossibly torrential rain from the cloud of condensing steam will finish the job.
Base 3, +3 sight, +1 part, +5 size
Muto Imaginem level 40 - Of Mice and Men
R: Sight D: Sun T: Group+3 (up to 10 000 men)
Changes the sound and appearance of an army up to 10000 men strong into that of an army of mice. Even if the victims realize it's just an illusion, it's essentially impossible for their leaders to coordinate them through sight and sound, completely destroying the command chain.
Base Level 2, +3 Sight, +2 Sun, +2 Group +3 size
Creo Auram Level 35 - Winterbreath
R: Sight D: Conc. T: Ind +4 size
A large bank of thick, impenetrable fog -- up to 10000 paces (about half a dozen miles) across -- raises from the ground. The mist blocks vision beyond a pace or so, and has a supernatural chill that saps strength from those within. Every round anyone in the cloud who fails a Stamina roll of 3+ loses a Fatigue Level (or takes a Light wound if already Unconscious).
Base 3, +3 Sight, +1 Conc, +4 size (Note: the "supernatural chill" is merely cosmetic, the effect -- and base level -- is identical to that of Wreaths of Foul Smoke).
Then just move to Base 3 (slightly unnatural) and chop off a size, there's room. And I don't think a bunch of medieval soldiers will drink water when they have just been provided with free liquor 
Creo(Rego) Animal level 35 -- Octavia's one thousand legions
R: Touch, D: Conc, T: Group +1 size
A vast army of wasps (5 million insects strong) erupts from the caster's mouth and flies to carry out a (potentially complex) task set by the caster - including destroying an opponent's army.
Base 5, +1 Touch, +1 Conc, +2 Group, +1 size, +1 Rego requisite
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Whilst I am not as familiar as many with all the rules about spell construction, I make it 3 magnitudes to achieve the 3 rego effects: one to bring a cloud down, one to move it around at will (possibly, you could combine these, but I'd still be inclined to keep 'moving clouds aorund' at 2 magnitudes), and one to make it move 'as fast as a running man'. It's still only L40, and I'd allow it.
I also see no reason for it to continue to affect birds after you've relinquished concentration - the whole spell should end at that point, why does the cloud stay toxic?
Several of the other suggested spells could fall under the guideline of 'that's a ritual because it's too powerful/out of paradigm'. I think anything that is actually described as 'mimicing the effect of a ritual' should also be a ritual - and all of these are not possible as an effect in an item. See p98 of the core rulebook, 3rd column, 3rd paragraph.
The problem isn't coming up with spells that slaughter thousands. No, the problem is rendering the existing ritual strike spells obsolete. Why would anyone ever learn/invent Neptune's Wrath (L40, ritual) with a size limit of 1 mile's width (which is stated as 'ritual for large effect') if you are allowed to make a spell like 'Instant Squall' with a size of 20 km? (square or linear, it isn't specified in the description.)
The more subtle effects using mentem and imaginem are fine, in my opinion, but most of these need SG approval to avoid being rituals.
Gilarius
(the party pooper - sorry)
This is an excellent point. Ultimately, it's up to the troupe to decide if a Formulaic spell should be a Ritual just because it is a sufficiently "imposing" effect, even if it meets all other requisites. Since this is not well defined, however, I think it's fair to provide the OP with the spells he's asked with the assumption that "imposing enough to wipe out an army" is not necessarily "imposing enough to warrant a Ritual".
Because it's a perdo effect - the cloud suffocates but is not toxic - once you've destroyed the vital spirit of a cloud it doesn't grow back.
Rendering the existing spells obsolete should be the goal of at least half of house Bonisagus and a good chunk of the rest of the order. In general you will get more bang for your buck if you destabilize a big chunk of nature and let the consequences flow than if you try to achieve your damage directly by magic. I predict that the Ignem spell is going to be one of the level 45 offerings for that very reason.
I wouldn't have a problem with a SG taking a look at Instant Squall and saying that must be a ritual but by the RAW I don't think they would have to (5+ size increases or 4+ and group target are automatically a ritual would be a good house rule). People would learn Neptune's Wrath because lab notes and teacher's for it exist within the Order. It is also far safer and more predictable WRT collateral damage.
It's a common problem, especially for Auaram magi, which is why, again, its so nice to know that as long as you avoid the limited durations and targets, there's no effect cap on the powers of Greater Enchanted Items. An oversight in the system? Perhaps. But one which my Verditius character has exploited with glee in the past.
English fleet? What English fleet? We never saw any English fleet, good Qaesitor. It must have been caught in a storm or something. Most unfortunate.
If your troupe does have a Verditius character, its worth investing in one insta-army crusher devise, especially one of the more "subtle" ones... giant storms do occassionally happen, after all. If nothing else, when paired with appropriate wards and Intellego spells, such storms provide great cover for desperate escapes. Otherwise, some magi with Celestial Magic can reach high enough lab totals for effects that are close enough for smaller forces.
Well there is this one from a recent thread. http://forum.atlas-games.com/viewtopic.php?f=4&t=6942&start=0
You could easily make this one a range sight duration concentration spell if you wanted as well but it just might be the case that the center of the effect is still the safest place to be.
So far we have:
Animal
Aquam (maybe too big but drop a couple of magnitudes and 20 hectares will cover a fleet at anchor if not strung out at sea)
Auram (x3, obviously the go-to form for this kind of thing)
Imagonem
Mentem
Terram
Vim
so Corpus, Herbam, Ignem still to go.
This would be even better with Harmless Magic:
When the duration ends, all that air pops back into existence and needs to expand outwards, sending a shockwave of furious winds rushing out to equalise the pressure (or, in Aristotilean terms, to find its natural place of rest.)
Talk about your double whammy.
If you must use Formulaic instead of Enchanted Items, I don't think that Ignem is going to happen, unfortunately, unless you get really creative and use a double-hit Columbae special using their unique Hermetic Virtue of Ring Magic.
Create a giant Columbae ring version of a CrIg spell. Can probably do +30 damage as the entire interior erupts in flames, but cast a MutoVim Holding spell on it until the army is within the defined space.
Ambush! 
The same can be done for Perdo Corpus, Rego Corpus and Muto Corpus.
Columbae held spells can be great as army killers, assuming they don't botch the circle.
Rego Corpus Level 35 - Curse of the faerie jig
R: Sight, D: Conc, T: Group+3 size
Makes an army of up to 10000 men lose control of their feet, which start shuffling, hopping, tip-tapping and prancing in a mad, furious -- but well-coordinated, and not unpleasant to the eye -- dance. While under the effects of this spell it is impossible to move around under one's own volition, except perhaps by pulling oneself along a rope. This mad dance is exhausting: every minute of dancing requires a roll for exhaustion (as per corebook, p.178 - Stamina-Encumbrance roll of 6+ to avoid losing a Fatigue level).
Base 2, +3 Sight, +1 Conc, +2 Group, +3 size
No need to use a high-damage Ignem spell. Just use a low-damage one with a big area as well as a longish duration like Sun (or even Diameter). That will slow-cook them.
No need to get so complex. Pit of the gapping earth PeTe15. Increase the size to +4 and make it sight. Drop the army into a 9ft deep pit. Works wonders with horses. Change the same spell to MuTe to change the earth into air make it concentration ( long enough for someone to fall 9 feet) and release. Massive grave.
I think the PeTe spell is 40 and the MuTe is 45.
Eh. If its a slow cook, they might flee or break the ring.
Better to flash-fry.
Also, less chance of any survivors to report things to nasty Qaesitors, who are always poking and prying where their noses don't belong... grumble glumble grumble...
In the words of Flambeau the Founder: No witness, no crime; no body, no murder.
I think the way to make Ignem work is to up the duration on a lower damage spell so the wounds build up and eventually kill (sooner or later there will be a high roll on the stress die.)
Enormous Enduring Arc of Fiery Ribbons (CrIg 45)
R: Voice, D: Concentration, T: Group +3
As Arc of Fiery Ribbons (ArM5, p140) but the fire extends 300 paces from the caster and continues to do +10 damage each round that the caster stands in place and maintains the spell. If cast with a Rego requisite the caster may change the direction of the arc in the later rounds.
base 5, +2 voice, +1 concentration, +2 group, +3 size
Or use Multicasting spell mastery, with a score of, say, 4 x mastery (16 copies) on a spell that splashes an area in sight with fire damage (Modify Arc of Firey ribbons to splash out from a point within voice/sight range, as opposed to from your hand). Repeat for 5 rounds. Enjoy roast marshmellows with your friends.
Actually i think the average army would reject the liquor(if for nothing else, because they would be well aware of the danger of drinking too strong liquor), or possibly dilute it with water, because this was a normal practise anyway, as it was safer to drink water with some alcohol in it.
As there´s lots of room in it for change, i would add a magnitude more to make it smell and taste like water.
Then it should work.
A cosmetic effect does NOT cause loss of fatigue and wounds!!! This is not a cosmetic effect. And yes its the same as WoFS basics, BUT you´re also adding one more "full" effect, the fog.
Add PeIg(or PeCo if the effect is only against humans) as requisites and ill say its somewhere close to fine (of course, then there´s formally the problem that their base for causing fatigue is 4 and 10 respectively).
Or raise it 1-2 Magnitudes as you´re doing 2 primary effects, both at the Base 3 guideline.
Not sure how well it would work, but darn i would REALLY like to see it used!
Base 2 is only "loosing control". What you´re doing above is "control a target´s motions" which is Base 10 or MAYBE the Base 4 "control the largescale physical movements of a target", although im a bit suspicious of the exact meaning of that phrasing. Still use the latter and its still at the requsted level 45.
Say WHAT?
And BTW Noliar, why level 45 and not level 50? If you´re aiming to max out the formulaic spells within RAW limits, thats level 50 before you must go ritual.