There are very few things in gaming that is worse than the dreaded Demonic Spider. These things in a nutshell are powerful enemies that are boss worthy but are in reality found amoung the common mook. And they tend to ruin the day of gamers should they get the chance.
So, in your experience, what are the most annoyingly lethal Mooks (i.e. nota character card) you've ever come across in Sentinels?
Glamour takes that spot for me. She will usually hit play and annoy the heck out of any character having a hard time making more than one hit a turn. I still consider her a mook cause she doesn't have a character card.
Citizen Truth comes to mind since it means I have to take him out regardless of how many other Citizens are in the trash already. He gets worse is Citizen Anvil was already out.
Sure, all the other answers here are ok. But when the thing you dread most, more than deamons or serial killers or cosmic entities that can destroy entire universes, are bubbles...well...
I always thought that Omnitron's Electro-Pulse Explosive card caused more panic than the boss its self. You pretty much have to deal with it as soon as it comes up and ignore everything else or you die.