Orcus. Demogorgon. Lolth. All three are extremely powerful foes. They graces the covers of Monster Manual 1, 2 and 3 respectively. These are the elite, showcase monsters of 4e D&D. They are among the toughest that 4e D&D has to offer. Few other creatures are as powerful or dangerous as these three. Defeating any one of them demonstrates that your party truly is the best ever.
And don’t forget about dragons. They too deserve inclusion on any list of the most powerful creatures in D&D. A level 30+ dragon is certainly a suitable foe for the climax of any epic tier adventure.
And then there is the Tarrasque. As I was browsing through the Monster Manual (as I’m wanton to do from time to time) I stumbled across the entry for the Tarrasque completely by accident. As soon as I saw it, I read over its stat block and I realized that Tarrasque has been overshadowed in 4e D&D by the monsters I described above. The creature once agreeably the most powerful monster in any Monster Manual is suddenly a forgotten footnote.