I read in the intro pages that your first SCP will probably suck and that's it's a good idea to get some input on your SCP before you post it, so here's my first couple of ideas.
A primate that can defy any security or containment procedure or system. It can essential break out of or into any thing it wants to. Naturally the greatest threat it poses is allowing info to leak out or to accidently release certain SCP's. It can also shape shift into various species of primates including a human, which told personell that there was one way to conatin it, they just had to find out what that is.
A snake inspired by the basilisk that can become essentially omnipotent in the absenjse of light. It can teleport, shapeshift, regenerate, and manipulate shadow in the absense of light. Even in broad daylight, it's still incredibly dangerous given it's crocodile armor, and potent venom. It also has a side ability to sense the location of anybody that is watching it, making it impossible to take by surprise. It physically resembles a cross between a pyton, cobra, frilled lizard, and crocodile.
Now theses are just off the top of my head so naturally they aren't very high quality if they are workable at all.