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.