This is not a question about what should be on the cover of a MM.As with the first 1e MM, there should be a few monsters on the cover rather than just one (it's a compendium, after all); and those could include any of Beholder, Dragon, Owlbear, Troll, Unicorn, Umber Hulk, and-or similar distinctive critters.
If you had to pick a single monster that represents D&D -- to you, to the market, to the fandom, to the public, whatever you want -- what would it be? What is, in your opinion, the quintessential D&D mascot monster?
Fair.Rust Monster.
Never pick a mascot that wants to eat you.