Quickling. Technically a Quickling only has 1 HD, but with natural invisibility, super high AC, multiple attacks per round, poison, and at-will spell-like abilities like shatter, dig, and forget, it could easily destroy a 1st-level party. Which is why it's worth 2000xp despite having only 1HD (enough for your fighter to immediately jump to level 2). Conversely, Planescape's Dabus has 4HD, but has poor AC (only 7), a single attack with mundane weapons, and no outstanding defensive capabilities or special attacks, and is thus worth only 75 experience points.
Obviously the XP value of a creature is a much better gauge of the creature's actual threat level. The various Monstrous Compendium volumes even included a detailed chart show how various special abilities, high armor class, and high damage output add to raw Hit Dice to generate the Experience Point Value of the creature.
Unfortunately, the numbers can be quite large and do not follow a linear progression (7 to multiple thousands), which makes quick eyeball comparison much harder than with Hit Dice (1 to 20). On the plus side, the chart does make it clear how the numbers line up.
The 3rd edition of Dungeons and Dragons introduced the concept of "Challenge Ratings" (CR), which directly tied amount of threat to amount of experience, while reflecting it in a simple 1 to 20 integer format (or 1/2, 1/4, or 1/8 for very weak monsters meant to be confronted in large groups). Using the chart provided in the Monstrous Manuals we can easily assign "Challenge Ratings" to monsters based on their XP Values.
Let's start at the bottom with one of the classic foes. 3rd-edition lists the Orc as "CR 1/2", meaning that 2 standard orcs should make a relatively fair fight for a 1st-level Fighter, which has been reasonably shown throughout most editions of the game. 2nd Edition gives the standard, no frills, baseline Orc and XP Value of "15". The chart shows that "XP 15" equates to "1-1 HD", which seems like a reasonable point to set our CR 1/2 at.
Obviously we cannot do a direct 1-for-1 comparison between 3e and 2e CRs for all creatures, as there are significant changes to the capabilities of many of them. Let's look at the next step up in generic humanoids. The next step up in the 2e Chart is "35 xp" for which the Gnoll is a good example, conveniently the Gnoll is also a CR 1 by 3e standards, so our chart maintains a close parallel. Thus CR 1 = 1 step up on the chart, and just slightly over 2x the experience of our CR 1/2 (15 x 2 != 35, but close enough).
So far so good, we can almost make a linear match, but what about the next step or two up? Let's look at the Azer. The 2nd and 3rd edition, Azers are practically identical (2+1 HD humanoid, immune to fire, slight magic resistance, +1 bonus to attack and damage from high Strength, heat attack that adds +1 fire damage to weapon strikes), and, in all, just slightly more powerful than a Gnoll. The 3rd-edition Azer is listed as a CR 2 (an appropriate challenge for a 2nd-level party), but the 2nd-edition Azer has an XP value of 420, 5 steps higher than our Gnoll, and yet, the two versions of the Azer have absolutely identical capabilities.
Let's try another 3rd-edition CR 2 monster. The hippogriff is also pretty much identical between editions, it has 3+3 hit dice, a good fly speed, an AC of 5/15, and 3 attacks for moderate damage. The 3rd-edition version is a CR 2, the same as the Azer, but the 2nd-edition version is only worth 175xp, less than half what the Azer is worth and 2 steps down on the chart. A quick look at other monsters listed as CR 2 in 3rd-edition shows a relatively tight range of XP values: 175xp for the 2+2 HD Sahuagin, 175xp for the 3HD Cheetah, 270xp for the 3HD Triton, and 270xp for the 3HD Wererat. Even the Bugbear which is the classic next-step-up in humanoids, comes in at 120xp, two steps up the chart from our 35xp CR 1.
From this we may have to assume a range of XP Values for each CR, rather than a 1-for-1 correlation with the chart above. This may be a good thing though, since there are published monsters that have XP Values that do not even appear on the chart, like Dark Sun's Bloodvine (50xp), Planescape/Chronomancer's Temporal Dog (375xp), and Dragonlance's Ursoi (775xp). Assigning a range for each CR will let us encompass creatures such as these that do not match the standard charts. So, looking at our sample CR 2 critters, it looks like most of them fall in the 120xp to 270xp range (we'll ignore the Azer as an outlier).
We can extrapolate ranges moving up, but where will the top-end be? 2nd-edition has some XP Values in excess of 100,000, but these are mostly for unique creatures like the Tarrasque, which should probably be ignored. Let's look at some classic apex monsters. The Pit Fiend and Balor are the top badasses of the Nine Hells and Abyss, respectively. They are both CR 20 in 3rd-edition and come in at 21000xp and 26000xp (respectively). Few other monsters fall into that power-level, but include Very Old red and gold dragons, at 21000xp and 22000xp respectively. The Great Wyrm gold dragon (the strongest of the standard D&D dragons) comes in at 25000xp. A Great Wyrm is a significantly greater threat than a dragon 2 ages lower, so we probably need to narrow the Pit Fiend/Balor gap and admit that the Balor is just stronger than his Hell-dwelling counterpart.
Let's set our CR 20 at a range of 24,000xp (Great Wyrm Red Dragon) to 26,000xp (Balors and Ultraloths), with catching the Great Gold Wyrm in the middle at 25,000xp. If we slot in the Pit Fiend as only one step below a Balor, then we can make CR 19 equal to 21,000xp to 23,000xp, catching the infamous Froghemoth, most of the rest of the Great Wyrm dragons, and the Balor's classic sidekick, the Marilith.
So here is our final break-down of 2nd-edition Challenge Ratings.
|CR||XP Value||Sample Creatures|
|1/4||10 or less||Cat, Kobold, Piranha, Weasel|
|1/2||11 to 30||Addazahr, Alchemy Plant, Goblin, Orc|
|1||31 to 99||Huge Centipede, Gnoll, Axebeak, Dabus|
|2||100 to 300||Bugbear, Centaur, Aballin, Ogre|
|3||301 to 500||Azer, Mephits, Giant Eagle, Shadow|
|4||501 to 750||Aranea, Cockatrice, Ghast, Eyewing|
|5||751 to 1000||Ursoi, Derro, Flail Snail, Venger|
|6||1001 to 1500||Nymph, Ravid, Son of Kyuss, Yak Man|
|7||1501 to 2000||Sylph, Doom Guard, Hydra, Grell|
|8||2001 to 3500||Spinagon, Cryo-Hydra, Xill, Great Wyrm Faerie Dragon|
|9||3501 to 5000||Dimensional Warper, Banshee, Bulette, Remorhaz|
|10||5001 to 6500||Kyton, Couatl, Death Knight, Fomorian|
|11||6501 to 8000||Sporebat, Behir, Frost Giant, Lich|
|12||8001 to 9500||Warden Beast, Aurumvorax, Blue Slaad, Mind Flayer|
|13||9501 to 11000||Roper, Noble Salamander, Roc, Juvenile Green Dragon|
|14||11001 to 12500||Night Hag, Glabrezu, Razhak, Dracosphinx|
|15||12501 to 14000||Beholder, Sword Archon, Storm Giant, Kraken|
|16||14001 to 15500||Nature Elemental, Retriever, Adult Copper Dragon, Astral Deva|
|17||15501 to 17999||Marid, Brass Minotaur, Marut, Psionic Lich|
|18||18000 to 20000||Phoenix, Animal Lords, Elder Orb Beholder, Nightwalker|
|19||21000 to 23000||Pit Fiend, Froghemoth, Titan, Death Slaad|
|20||24000 to 26000||Balor, Ultraloth, Great Wyrm Red Dragon, Great Wyrm Gold Dragon|
|21||27000 to 30000||Colossus, Dregoth, Hephaeston, Wight King|
|22||31000 to 35000||Ghost Dragon, Solar, Corpse Tearer Linnorm|
|HFS!||36000+||Graz’zt, The Dragon of Tyr, The Tarrasque|
You can get a spread-sheet of all of the monsters in AD&D 2nd edition, filterable by hit dice, XP value, and our new Challenge Ratings at this link and can look at stats for all published 2nd edition monsters on the Lomion Monster Index.