"Jeepers, Creepers, where's Peepers." This episode is still disturbing to me. The way Koosie's dog started becoming grotesque while being ruthlessly experimented on, and the fact there was an anime styled villain in a cartoon show was always weird to me. But, it still remains one of my favorite episodes.
"Dexter's Rude Removal". Enuff said, no one can argue with me on this as this is the most gruesome, evil and devastating Dexter's Lab Episode ever made. ps. Its banned in every country, and has never been aired ever (except once in Singapore or some other foreign country) and is played in private parties in mature audience environment back in the 90s in the USA.