Clearly, the correct answer is Giant Frogs. They are the perfect embodiment of the weird and whimsical Gygaxian style... even if they first appeared in Dave Arneson's Blackmoor! It isn't easy choosing between the greats - gelatinous cubes, green slime, rust monsters, beholders and such come to my mind - but in the end, if I have to go with one choice, it has to be the frogs!
As they hungrily dig for halflings in their burrows
It was a running gag to. I was playing a halfling thief and the DM got a kick out the statement in MM so every time I played a halfling he'd use the statement above.
By god, I can take out the most powerful party ever assembled with a hungry swarm of stirges. And my love for these creatures was made tenfold when Ed Greenwood did his ecology on them. My favorite of all time, and my players know it.
Harpies. They were illustrated in my old blue box rules with big, bare breasts.
That makes quite an impact on an 8 year old who hasn't seen nudes before.