"Hmmmm. Many bandwagon pass this way kimosabe."
I have killed more characters with these screeching little maniacs than I've had hot dinners. Heck, I've killed characters in sci-fi games with goblins. WFRP. WowCraft. Rokugani. Pathfinder. Whatever your flavour, everthing's better with added goblins!
9: Blue Dragons
What's scarier than a dragon breathing fire on your poor sorry earthbound monkey butt? A dragon that shoots lightning from its mouth (and probably fireballs out its' erse). Blue dragons are living thunderstorms. Consult your inner geek: you know they're cool.
8: Winter Wolves
I always loved the Norse myths, and there's just something primal about these guys. A slavering pack of giant wolves that hunt you down across foggy moors and then literally freeze your breath in your throat.
7: Fire Giants
Another favourite from Norse myth. The sons of Surtr. They spend their days in volcano forges crafting the weapons they will use to crush the world of men. They sound like the Isengard music from the LOTR movies, and they want to burn the world.
6: Iron Golem
As animated by Ray Harryhausen. Breathes poison gas and crushes all in its path. Yeah, it's just that awesome.
The Dragger Down. The dweller in the depths. The stuff of nightmares. If you're being all "MM1 only!" pedantic about it then substitute Giant Squid/Octopus. Either way, malicious giant cephalopods ftw.
Vampires? Too Bela Lugosi for my tastes. Liches? If I wanted deathly, foul-smelling bibliophiles I'd go to the FLGS. Zombies? Dialogue is too limited. The ancient dead were first and are still the best. Plus they have the best aesthetic. All hail King Tut!
3: Type III Demon
It's a giant dog-headed demon with pincer arms sprouting from its chest. I thought it was cool when I was 13, I still think it's cool now. The Type III is what every blood-soaked idol in a smoky cult temple should look like.
(The DiTerlizzi illustration in the "Planescape" monster book did the beast no justice and does. not. exist. Got it?)
I first read about these guys in the 1E Dungeoneer's Survival Guide. I was hooked. Then I read their monster write-up in the MM2. I had a new favourite ancient evil from the depth. Illithids? Kuo-Toa? Sorry, who?
Pick out your 1E Monster Manual. Open to the Sahuagin entry. Read it. There are highly-intelligent evil devil-worshipping mutant Atlanteans living in the oceans, and they like to farm the coast dwellers like humans do fish. They have whole kingdoms down there, and they're smarter than us.
Read China Mieville's "The Scar". His grindylow <==> my sahuagin.