D&D's influence over what it means to be a 'monster' in gaming is broad and profound.
Top awards have to go to:
Beholder - The 'eye tyrant' with the power to cause many magical effects with its gaze is an amazing monster, and a worthy and feared foe.
Mind Flayer - Extremely alien and horrible.
Dragon (D&D variant) - D&D didn't invent the dragon, and has never been comfortable with its chromatic variation on them, but managed to stamp its impression on dragonness despite that.
Orc (D&D variant) - D&D didn't invent the Orc, but in its bungled attempt at Tolkien orcs managed to invent something that seems to have more ressonance. Between the orc and Star Trek's Klingons, we've basically defined what it means to be a warrior race not valued for thier personal appearance. Oddly, the Orc has become so universal, that you can no longer make a D&D shaped stamp by using one. Serious dungeon-heads prefer goblins, hobgoblins, gnolls, bugbears, or kobolds to the ubiquitous orc.
Elf (D&D variant) - D&D didn't invent the elf, but in its bungled attempt at Tolkien elves, managed to invent something slightly more comfortable and playable than Tolkiens immortal uber-people.
Dark Elf (D&D variant) - D&D didn't invent the Drow, but you can hardly be involved with fantasy anything without tripping over D&D's interpretation of them.
Roper - The lesser known portion of the D&D's trio of 'bottom of the dungeon' defining dungeon dwellers.
Other Classic D&Disms:
Owlbear - This and the next five just reek D&D to me. In a good way.

Green Slime
Yellow Mold
Bugbear (D&D variant)
Rust Monster
Carrion Crawler
Deadly Pudding
Troglodyte
Kopru (In the midst of a big breakout to the big time, long overdue)
Sons of Kyuss (Recently had a big breakout to the big time, long overdue)
Mephit
Gibbering Mouther
Aboleth
D&Dism seemingly on the wane
Blink Dogs
Displacer Beasts
Hook Horrors
Monsters I've been rooting for
Slaad
Dark Creeper
Grill
Varguille
Phase Spider
Mimir
Those along with animals, vermin, deadly plants, undead, fairies, and the classic monsters of myth and legend make up the vast majority of my pallette.