The methodology I've settled on is as follows:
- The creature is the dominant inhabitant of the correct alignment plane (according to the Manual of the Planes). The Greyhawk (default setting for D&D 3.5) cosmology lists the Outer Planes that are keyed to the nine alignments.
- The creature has the appropriate alignment subtypes - an iconic Lawful Good creature is both (Lawful) and (Good). A neutral creature obviously doesn't need to be (Neutral) because that doesn't exist.
- The creature is strong. Wouldn't it be lame if the ultimate destiny of your soul is... being shaped like an imp?
- The creature is not obscure, appearing in the core books of D&D 3.5 and throughout the history of D&D. A creature can't be iconic if it's buried in the Dusty Tome of Whatever.
Lawful Good - Trumpet Archon
Mount Celestia is the plane that is in equal parts Lawful and Good. Its native sons and predominant denizens are Archons - angelic beings of Law and Good that are sent to battle evil throughout the planes. There are many kinds of archons, but of the ones in the Monster Manual, the trumpet archons are the mightiest. They hearken all the way back to the very first Manual of the Planes published in 1987, meaning they're older than a good chunk of people that play the game!
Neutral Good - Leonal
Elysium is the plane of pure Good, balanced between Law and Chaos. Its dominant inhabitants are called guardinals, of which Leonals are the strongest in the Monster Manual. These guys hearken back to 1995. Surprised that angels aren't here? Turns out astral devas, planetars, and solars are spread evenly between the Good planes (alignment: always good (any)), rather than being properly Neutral Good.
Chaotic Good - Ghaele Eladrin
Arborea is the plane of choice for Chaotic Good beings, and eladrin are the Chaotic Good beings of choice for D&D. Manual of the Planes literally says that ghaele are the "best known" of the eladrin (p. 145) so there you go. They go back to 1995 like the leonals.
Lawful Neutral - Marut
The most Lawful plane in the cosmology is Mechanus. The MotP describes Inevitables as their primary residents - beings so Lawful that they go around enforcing the laws of the universe! The oldest (and strongest) of these is the Marut, who enforce death. They appeared in 1987 alongside archons. Technically, Modrons are older Lawful creatures, appearing in 1983, but they got almost no attention in 3rd and 4th editions.
True Neutral - Tarrasque
This is sort of tricky - the Outlands are more of a meeting ground for planar powers than a thing in its own right. The Rilmani are described as the True Neutral outsiders in a few obscure books, but they hardly have the notoriety to be called truly iconic. Creatures that are too dumb to have ethics and morals are also True Neutral, so you could pick the Tarrasque (the biggest, baddest of the beasts) as the paragon. He came out in 1983 and is an iconic high-level enemy. He's not iconic for True Neutral, but he's the most iconic creature that is True Neutral, for what that's worth.
Chaotic Neutral - Gray Slaad
Slaadi are the best-known monsters of Limbo, the plane of true chaos. The MotP says they have been natives "since time began" (p. 94). The slaadi are so iconic that they are product identity of Wizards of the Coast, and not licensed out as part of the Open Game License. Introduced in 1981, these frog-like outsiders are even older than the archons.
Lawful Evil - Pit Fiend
When it comes to the kings of Baator, there's only one race that rules over all others in the Hells. Devils! Baatezu! By any name, they are the iconic Lawful Evil beings - fighting the Blood War on the side of Law, and causing Evil acts in the world. Pit fiends are the generals of Baator's mighty armies. They are both the strongest beings I've listed so far, and the oldest, appearing as far back as 1977.
Neutral Evil - Night Hag
The Gray Waste of Hades is where the Blood War between Lawful and Chaotic Evil takes place, so there's not a lot of iconic creatures here that are still alive. Yugoloths (or daemons) live here but they are from Gehenna, a different plane. These guys were never really popular, unlike Night Hags, who appeared in 1977's Monster Manual and were said to rule in Hades. The hag is also substantially distinct from demons and devils, which is a nice bonus.
Chaotic Evil - Balor
If the devils are classic LE, demons (tanar'ri) are classic CE. They go back to 1976. Ruling the Abyss, the quintessential Chaotic Evil plane, demon armies are led by the Balors into battle against devils, in the eternal Blood War between law and chaos.