This is another one of those "how do I pick just one?" categories. So I will list a few and try to make up my mind. Choices, choices...
Orc: Ah, the indispensable orc. How many villages have they raided, how many caravans intercepted, how many mines overrun by orcses? Pig-faced or otherwise, they have lived and died in countless adventures, and have leveled up many a young adventurer through their low levels.
Lizardman: The Trampier illustration in the MM definitely made an impression on me years and years ago. I've always thought of lizardmen as interesting enemies, and they were a staple in my dungeons and adventures when I was a young DM.
Gnoll: Another humanoid that captured my attention at an early age. Stronger than orcs, and their feral appearance only adds to their appeal.
Sahaugin: These aquatic raiders are hardy and versatile. Only the kuo-toa are more deadly and feared by adventuring parties, at least in my opinion.
Hobgoblin: I actually prefer hobgobs to orcs in my adventures. It must be the fact that they are militaristic, take care of their armor and weapons, and hate elves. And with their natural proficiency at mining, that makes them an enemy to dwarves as well!
Drow: From the pages of the MM: "The 'Black Elves,' or drow, are only legend. They purportedly dwell deep beneath the surface in a strange subterranean realm. The drow are said to be as dark as faeries are bright and as evil as the latter are good. Tales picture them as weak fighters but strong magic-users."
OMG what an awesome teaser for the G-D-Q series! Who would have known at the end of G3 that the adventure was just getting started? And that once let loose, the drow would become one of the most feared and hated enemies of all time? So my pick are the drow, hands down, no debate, easy peasy.