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Ah yes, the gelatinous cube . . . a monster bred for the 10'x10' passageways of dungeons. I too voted for the G.C., Gygax's answer to Lovecraft's Shoggoth.
Since Flumph wasnt on the list, i had to go with the monster that has been a giant part of my campaign, the Gelatinous cube. One of the PCs in my game has a mini cube as a familiar. It's name is Smuckers =)
I voted Mind Flayer, since it's the only one IMHO that you can write something about, or even have a whole campaign evolve around a single one as a villain or a whole group.
Every other monster in the list is somehow disposable dungeon junk Well, it's useful indeed, it's just that I never managed to do much else with them except a quick fight in the middle of a dungeon (or forest). The beholder is definitely a D&D signature monster, but I never liked too much its computergame-like array of stuff.