What's the heaviest defeat the Romans suffered at the hands of Greeks?

I failed to recall any great battle in which the Greeks/Hellenistic kingdoms, maybe except Pontus which is only half Hellenistic at the most, inflicted a crashing defeat upon the Romans, the closest were Pyrrhos with his Pyrrhic victories. While almost every other people who ever fought the Romans in the same period,i.e. Carthaginians, Britons, Spaniards, Numidians, Germans, Gauls, Parthians, etc. could boast of at least one Roman army totally destroyed by them.
If my impression is reasonably accurate, why were the Hellenistic armies so ineffective against the Romans?

First of all, you shouldn't dismiss Pyrrhos' victories out of hand, just because of our modern association with the name. At the time, they were clear victories. It's just that they did not result in the submission of Rome. Conventionally, the side that was defeated in a large-scale battle would seek a truce. Rome, however, refused to play by the book and simply gathered another army. They could afford to lose, whereas Pyrrhos, who wasn't in home territory and couldn't draw on a large population base, could not. The Carthaginians faced the same problem during the Punic Wars.

In any case, Pyrrhos won two battles against Rome and lost one. Subsequently, he went campaigning in Sicily, Macedon and Southern Greece. I doubt that would have been possible if he had suffered substantial losses in both his victories against the Romans. If anything, it suggests his defeat didn't much impair his ability to wage war. Rome also refrained from attacking Tarentum (his Italian ally) until after he died.

However, you are right that the later Successor Empires fared very badly against Rome. This can be attributed at least partially to the fact that these conflicts occurred a decade or two after the Second Punic War. Rome had mobilised staggering amounts of manpower to defeat Carthage, so in the decades that followed, it could draw upon a enormous pool of veteran soldiers and officers. During this time span, Roman armies were practically invincible. However, as soon as this generation retired, Rome suffered a string of defeats at the hands of barbarian tribes and the sorry remains of Carthage.

That came too late for the Successor Empires, though, which had already been conquered or substantially reduced. They would never again be able to face Rome on equal terms.

Well, I didn't mean that Pyrrhos's victories were only half victories or something like that, only that his victories, being the best performance the Hellens ever had over Rome, were still in no way near Cannae, Carrhae or Teutonic Forest.
I agree the successor kingdoms were unfortunate that they had to face Rome at the worst possible time for them. But considering almost any barbarian tribe and, as you put it, even "the sorry remain of Carthage" could teach the Romans a lesson on the consequence of arrogance and rashness, the Hellenistc world looked simply too bad in comparison.
My opinion is that it has something to do with the facts that they were too close to the Romans, both geographically and culturally. And whatever they excel in, the Romans are better, so it's almost impossible to play their strength against Rome's weakness, while the Nomads have their deserts and HAs, the Gauls their shock troops, the Germans their forests, the Spaniards their mountains and skirmishers.

Both Greeks and Hellenes had declined in military power before Rome encountered them. The Greeks never recovered from being smashed by the Macedonians, and the Hellenic Successors all suffered from depleted Hellenic populations due to decades of constant warfare.

the romans suffered heavily in iberia 7 legions destroyed in 10 years against viriathus 2 legions destroyed against the numantines and had it not been for one of the grachus a 3rd one would have happened