The most sinister troll I have is a psychic grub a la Brain from "Pinky and the Brain"
Here's a hint, though: Just because something looks creepy or you say "THIS IS CREEPY" doesn't actually make it creepy. There's a very specific art to being unnerving. There's also a range of responses to unsettling images/characters. Some people, myself included, have a really high tolerance for creep. Some people can't sleep the same night they learn about Slenderman.
Some general advice, though: The closer something is to human without actually being human, the scarier it is. I'm pretty sure there's some term for this, I think it's "uncanny valley", but that's dumb. Basically we're scared of things that are familiar but different. That's why serial killers are so scary. They look like humans, but inside their heads there is absolutely no traces of empathy or any other emotion that defines us as human beings. This is also why corpses are scary. They are humans, but they aren't alive and they remind us of death, or squick us out.
Basically, in order for a troll to be good and creepy you can't just give it bleeding eyes or make it say scary things or laugh a lot. A good creepy troll has to be seemingly normal. They have to feel familiar and harmless, but have something about them that is unnatural.
I'd like to bring up Gamzee now for a second. You might say "hey but he's fucking nuts and HE'S scary!" Yes, that's true, but that's because he wasn't always that way. He was always a pretty benign and normal troll. He had a lusus, he chatted with his friends, he had his various interests and he could club an imp if need be. What makes him FUCKING TERRIFYING now is that he no longer seems like a troll, but he still is. He lost all traces of his empathy, his personality, everything that made him the adorable goof that he was. Another thing that makes him scary is that he has a goal. I've noticed a lot of KILLDEATHDESTROY trolls here are just angry and murderous because they are or because they have some really stupid quirk to them like they vomit hornets or their brains were fucked into a pile of sludge by a horrorterror at birth. Gamzee is following his religion. He came to the revelation that he wanted to kill because of that, it wasn't because he was ALWAYS sinister, it was because he snapped like a twig during a crisis of faith.
Now everyone has something that freaks them out, no one is immune to fear. The thing is, in order to be scary to others, you have to be able to know what scares you. Take a look at these pictures, they are all things I have collected that are different types of creepy. (I love creepy, I absolutely love it) There is a chance none of these will faze you at, and there's a chance it will make you feel really uncomfortable so I'm warning you now: the third picture is incredibly gorey and completely NSFW. Do NOT open it if you are uncomfortable at the sight of gore.
Now, if any of those images squicked you, or made you uneasy, try to pinpoint exactly what it was that did it. If you can understand precisely what is that made you feel unpleasant, then you can start to explore using that in your own work. What scares you might not scare anyone, but if you make something working from your own fears, you will make it scarier subconsciously. It's because you're designing it in the specific way that gets inside your head and makes you cringe. When you try to make something that scares someone else, you are just kind of blindly throwing "scary" aspects together and most of the time, it will just fall flat.
This ended up being way more ramble-y than I had anticipated. I hope someone got something out of this, because I would love to see some real, bonafide, gen-u-ine creepy trolls around this thread.
Oh and remember! The more BACKGROUND STORY you write about the troll, the creepier it is! By putting thought into the character, you can take something that would be just another "meh that's kind of creepy looking" troll and make it really unsettling! I think ThunderReign does a really great job of this, as their trolls are always really well flushed out to give them that extra squick factor.
WELP. I think I've said a whole lot, and I'm not sure if any of it will make sense.
Happy slumming, fellow troll creators!