- Elementals (8-16 HD)
- Purple Worms (15 HD)
- Giants (8-12)
- Dragons (5-12)
- Hydras (5-12)
- Balrogs (10)
- Efreet (10)
- Black Pudding (10)
At the very top of the list are Elementals, in particular the 16 Hit Die types brought about by the 5th-level magic-user spell, conjure elemental, and are subject to a whole slew of special restrictions and risks (only one per type per day, large raw material requirement, maintain concentration or caster gets attacked, etc.) You can see how important those limitations are, when you get to call forth the toughest monster in the game-world, and put it under your control, any time you like (and it's not even the highest-level spell).
Other than that, the only thing more powerful than Giants or Dragons are the tremendous Purple Worms. Actually, I really like that flavor -- the most dangerous creature in the natural world, blind in the underworld, burrowing incessantly "just beneath the surface of the land" (Vol. 2, p. 15; compare, for example, to the monstrous "Dholes" in various H.P. Lovecraft stories).
Balrogs, of course (item #6), were included in the earliest editions of the OD&D game, but were removed in later printings after a skirmish with the Tolkien estate. They reappeared later on in Eldritch Wizardry as "Demon, Type VI (Balrog)" (Sup-III, p. 12), with some minor name-mangling afterward in the AD&D version. (See more here.)
And the other thing that might be surprising is the appearance of the Black Pudding monster, described as just "another member of the clean-up crew and nuisance monster" (Vol. 2, p. 19). Note the extraordinary strength of this creature, as shown by its very high Hit Dice (and brutal 3 dice of damage, the most in the game!) Perhaps the creature either needs to be as enormous as other creatures listed here, or be extremely rare due to some supernatural or unearthly part of its makeup. See the adjacent picture; scary! [from Sup-I, Greyhawk, p. 14] Other ooze-types like the ochre jelly, green slime, gray ooze, or yellow mold have only a fraction of the same hit dice. (Note also that, oddly, only the ochre jelly appears on the wandering monster tables.)
The list is modified somewhat if you take into account the special hit point accumulation for Dragons by maturity. In particular, a Very Old (6 hp/die) Gold dragon has 12 × 6 = 72 hit points, which is on average the same as 72/3.5 = 20 hit dice. In other words, it's the most powerful creature in the game (unless there's a great-granddaddy Elemental or Purple Worm somewhere that rolled all 5's and 6's for its hit points). The same can be said for Hydras, of course.
In addition, there are notes in the text that indicate the possible existence of even more powerful creatures. Sea Monsters start as Purple Worms, and increase to 2 or 3 times that size! (30 to 45 hit dice; Vol. 2, p. 15; although reduced from those levels when they later appeared as prehistoric beasts in Sup-II Blackmoor.) Rocs start at 6HD, but can increase to 2 or 3 times the basic listing (up to 18 HD, which became standard in AD&D; p. 17). Animals are also considered up to a Tyrannosaurus with 20 HD (p. 20).
Finally, If you add Supplement I: Greyhawk, then there are other rare and powerful monsters. These include Titans (effective 25 HD), Golems (23, 17, or 11 effective HD), Storm Giants (15), Giant Slugs (12), Beholders (11 or so), and Liches (10+). Note that some of these creatures are actually proposed in the original set (Vol. 2, p. 21), along with ideas for super-strong Cyclopes, Juggernauts, Robots, etc.
Honorable mention goes to the following (8-9 Hit Dice): Vampires, Gorgons, Chimeras, Treants, and Invisible Stalkers (plus Will O'Wisps and Umber Hulks from Greyhawk).