The Godforged Colossus
4th edition Monster Manual page 44. Look it up.
If you are within 5 squares of the Godforged Colossus at the start of your turn, you take 20 psychic damage. This immortal construct is a BEAST! Ok, he’s a Level 29 encounter, but my goodness it is so sexy. I have never fought one, but I really really want to. I want to see if a high-level party would be amused, challenged, or thrashed by this monster of a monster. Its abilities mean that it can dominate in close combat, and Voice of the Demiurge can handle all ranged attacks.
Just read it. And weep. Ok, the Tarrasque is tougher, but you could have SEVERAL Godforged Colossi in a single encounter. Terrifying.