As stated in the title, what official monsters have you run or encountered which seemed to be far stronger or far weaker than what their listed CR would lead you to believe 

The archmage should be at least one cr higher, in my opinion.

At CR1-3, they are usually pretty much fine. After CR3, almost all monsters are far too weak for their CR level. CR4-9 monsters have about 1/2 the hit points they need. CR10 monsters have about 1/3 the hitpoints they need. Adult Dragons with 200 hit points are laughable.

Notable mentions go to:

banshees at CR4 can TPK a whole party with one ability.

intellect devourers are ridiculous and should not be used by anybody. First time I encountered one as a level 6 character, I rolled poorly and was instantly killed. I had no idea what they did. There is no way outside a Wish spell (maybe a generous DM might allow greater restoration that I know of that can restore intelligence points.

catoblepas is another stupidly designed monster with an instant death ability.

vampires are a bit of a joke. Their damage output is crazily low.

gibbering mouthers are problematic since there are no death saves if it takes you down, and if it scores a crit with its 17 (5d6) damage bite then it will one shot most adventurers who are up against it. No creature should auto-kill just because they rolled a nat 20, but it basically does.

gynosphinx and androsphinx are CR11 and CR17. The Androsphinx is probably ok because adventurers facing CR17 creatures might have the means to resolve things, but the gynosphinx can move time ten years into the future, effectively completely disrupting the campaign and only a wish (which level 8 adventurers, for whom this is a reasonable encounter) probably don't have access to.

The legendary kraken really isn't all it's meant to be for a CR23 creature but at least it has a decent amount of hit points.

mind flayers are interesting. They're overpowered at CR7 if they have helpful allies, since their stunning blast is likely to cause the heroes to really get wailed upon, and if you're against multiple mind flayers (3+) then that can be devastating, especially with their brain eating. However, on its own a CR7 Mind Flayer isn't all that tough and gets nuked in a single round.

Shadows are notorious TPKers since their attacks can kill a character in as little as 2 hits as well as reducing the accuracy and damage of any STR-based attackers.

Pixies also break the CR math with high level spells. Yeah, they're only 1hp but that doesn't stop them from casting polymorph and then running away.

i've found genies to be laughably underpowered for their CR.  Their best spells are limited in many ways by concentration, their attacks don't do a lot of damage, and even though they can fly, their best attacks and abilities require melee or very short range, so if they want to engage in combat, they have to move in close.  Their best skill is escaping, not dealing damage or defending.

Monsters with trick powers that the PCs can shut down easily, such as regeneration, have that power counted into their CR as if it doesn't ever get shut down, so they tend to be underpowered against PCs who know the trick or accidentally are using appropriate powers (I mean, what are the odds that a higher level party isn't using radiant damage).

Quote from DennisThePeasant >>

Set up a Fight with three CR 1/2 Hobgoblins against four 1st level chars. Assume the Hob's get Initiative, and focus fire on a single char, and the DM plays them to their fullest abilities, with Martial Advantage. Watch what happens.

I put two 5th level chars up against two CR 1 Scarecrows. One of the players, who had 61 HP, escaped with 6 HP. That Frighten/Paralyze ability is devastating. 

Even with just two Hobgoblins that's a Hard encounter for a party of four 1st levels, with three of them it's well over the Deadly threshold. As for the Scarecrows, it's a single target ability with a DC 11 Will save - that's almost a coin flip even for characters with a perfectly average Wis and no proficiency - and the paralysis only applies to the Glare, not to the claw attacks. The issue is that what drives a combat encounter's momentum is the action economy, which is why encounters are calibrated around a party size of 3-5; having fewer PCs than that is always going to throw things out of whack. Scarecrows are rough, but vs a regular size party as much because of the resistances against non-magical damage as because of the debuffs.

Quote from Sanvael >>

the gynosphinx can move time ten years into the future, effectively completely disrupting the campaign and only a wish (which level 8 adventurers, for whom this is a reasonable encounter) probably don't have access to.

 This effect does not reach beyond the confines of the lair, so it's not that crazy considering there are no aging/de-aging mechanics. It boils down to either pure RP or "the DM decided you should die with no save or recourse beyond wish."

Quote from DennisThePeasant >>

Quote from pangurjan >>

Quote from DennisThePeasant >>

Set up a Fight with three CR 1/2 Hobgoblins against four 1st level chars. Assume the Hob's get Initiative, and focus fire on a single char, and the DM plays them to their fullest abilities, with Martial Advantage. Watch what happens.

I put two 5th level chars up against two CR 1 Scarecrows. One of the players, who had 61 HP, escaped with 6 HP. That Frighten/Paralyze ability is devastating. 

Even with just two Hobgoblins that's a Hard encounter for a party of four 1st levels, with three of them it's well over the Deadly threshold. As for the Scarecrows, it's a single target ability with a DC 11 Will save - that's almost a coin flip even for characters with a perfectly average Wis and no proficiency - and the paralysis only applies to the Glare, not to the claw attacks. The issue is that what drives a combat encounter's momentum is the action economy, which is why encounters are calibrated around a party size of 3-5; having fewer PCs than that is always going to throw things out of whack. Scarecrows are rough, but vs a regular size party as much because of the resistances against non-magical damage as because of the debuffs.

 I agree with your assessment, but the fact remains those monsters fight way above their weight class.

My assessment is they don't, so I'm not sure what to make of that. :p Regardless, agree to disagree. 

Quote from DennisThePeasant >>

I agree with your assessment, but the fact remains those monsters fight way above their weight class.

 Hobgoblins? Sure, martial advantage gives them high damage, but their accuracy stinks and their hit points are terrible. A Thug has higher average damage (due to pack tactics and two attacks) and better durability (ac 11/32 hp is tougher than ac 18/11 hp).