What do consider to be a Powerful Monster...in a normal game?

[Rerednaw]

In a standard party of 4 15 point buys. Or say using 4 of the pregens. Which monsters do you consider to be above the standard for their CR? Obviously combinations or sheer numbers for action economy can also be factored in...but I am focusing on individual or say a pair.

Here are a few off the top of my head...
Any intelligent monster...played smart.
Swarms...mainly at lower levels.
Most creatures using the young template.
Sandman. Sleep aura, sleep on hit.
Witchfire. Incorporeal, fly perfect, touch attack, ranged touch, grants vulnerability...let me count the ways...
Air Elemental. Whirlwind, pick up the party, fly up, drop from top of whirlwind. Repeat.
Ghoul with a rogue level. In an intro mod...you know which one :)
Shadow Demon...all this and magic jar for CR 7.
Burning Skeletons. Works great against pouncing Eidolons at low levels.

[Rynjin]

Swarms and Incorporeal creatures are a given. Most dragons are tough for their CR.

Anything that can have both powerful spellcasting and a solid melee presence (such as the aforementioned dragons, higher CR Outsiders, etc.)

Celestials, who pretty much rock Fiends of a similar CR. Which is frustrating in an evil campaign.

[Victor Zajic]

Large Earth Elementals

[BigDTBone]

T-Rex

[Zilvar2k11]

Assuming that by 'standard game' and reference the pregens, you're talking about something pretty different than most of the posters here seem to consider 'standard'....

Anything with earthglide/tremorsense, or druid levels for passing through plants.

More generally, anything that can attack the party from any point and the party can only respond by readying actions and/or getting away from the dangerous area. Druids are particularly nasty with the combination of entangle (to make rough terrain) and pass through plants so they can't be charged if they happen to leave themselves vulnerable because of spellcasting.

Anything with multiple status effects and decent to-hit rolls (ghouls being the primary low-level offender to me). I do not care for these creatures because they interfere with player agency. If, in the middle of a fight, you might as well just get up and watch TV for the remainder it's a poorly designed monster and/or ability.

Anything that requires magical or special weapons to damage and is CR'd earlier than you'd reasonably be expected to have those things (level 4 for a +1 sword for example, even though an argument can be made that people should be stacking potions by level 2). This is an invitation to frustration and encourages players to focus spend money on magic weapons as early as midway through level 2, because +1 sword is easier to remember, use, and book-keep than '# vials of oil of magic weapon (CL1), # vials of silversheen, # vials of ....'

Essentially, anything that interferes with players being able to do something, or anything that encourages players to ignore the game guidelines on advancement, is a Powerful Monster in a hypothetical Normal Game.

[Mydrrin]

Anything the fighter has to roll a 15 or better to hit. Fighter should be hitting things on a 10-12 for most things.

Anything that has a DR that can't be overcome by your party or will make downing them near impossible.

Nobody should feel completely useless on fights. That they can hit you but you can't hit/affect them.

Very tough save or be messed up too many times in a row isn't fun. Everyone wants to be a hero, you are telling a story, don't make it painful work.

[Daenar]

Myrrdrin, a long time ago my circle came to a conclusion that anything the fighter needs to roll a 14 or more to hit, should warrant careful gm consideration before use. This was used as the gauge for boss ac values.

[kBro]

Phase Spiders

Yea, lets just sit in the ethereal plane, pop into the material plane, bite you for con poison, and shift back and wait until you die.