When counting down the most powerful DC cosmic characters of all-time, the number one on the list has to be DC's version of God. This being is known as The Presence. While there are numerous Gods in DC, mostly from the Greek versions of Zeus and his offspring, the Presence is the Christian version of the character.
The Presence is the being that gave the Spectre his powers -- which means he created one of the most powerful beings to ever exist in DC Comics. He has the power to resurrect the dead as he did with the Justice Society. The Presence appeared in Lucifer and showed he was not absolutely omnipotent, and since DC and Vertigo merged, that remains the case for now.
The Guardians of the Universe are the immortal founders and leaders of the Green Lantern Corps. Their goal in life is to bring order and truth to the universe and do it by any means necessary. While they are not as physically strong as other beings on this list, they are possibly the most intelligent beings in existence -- and true power lies in knowledge.
Their powers include durability, awareness, vast knowledge, and a psionic ability through their own immense willpower. Their psionic abilities are even more powerful than the strongest Lantern's, as Kyle Rayner said in Green Lantern Vol. 3 #100 that the Guardian Ganthet could crack a planet in half with a simple thought and he also staggered Anti-Monitor with a psionic attack.