The Markovians, from the Well World series.
These beings reached the pinnacle of technology in their universe, finally deciphering the underlying math that creates the universe itself and making planetary-sized computers capable of altering said math. Thought-controlled computers, mind. Which meant that every Markovian was essentially a full-out reality warper, protected by those very computers, literally impossible to kill whatever the force levelled against them, due to the web of computers across their universe supporting their own 'math'. Immortal, all-powerful, in full and total control over their universe.
Then they got bored.
They, themselves, created the Well World, another computer far greater than anything they'd done before. They then seeded the Well World with their own selves, limiting themselves and changing their shapes in an attempt to create the seeds of life for a new universe (and cutting the vast majority of their race off from their reality controlling computers). The sole Markovian left as caretaker to the Well World used that computer to bust open a white hole into another universe, effectively opening up more stuff he could take and mold with the power of the Well World.
He used that power to make a new universe atop the old one - a bigger one, better one - and seeded it with his people (in their altered forms) all across this new universe, creating new life.
Billions of years later, something went wrong. He went back to the Well World, used the computer to wipe the new universe, then started over. Created, used his own mind to control the Universe's growth, then used the Well World's 1560 races (all descendants of the original altered Markovians, or 'entries' - people who 'fell into' a Well Gate and were changed into another race and placed upon the Well World - it's a confusing place, don't try to understand it) to repopulate again as seeds.
This happened seven times. Each time, new universe.
And, as before, the caretaker is impossible to kill as the Well World, a universe-controlling computer easily capable of punching holes to other universes and controlling the vast flood of creation into going where its user wants to go, protects him when he's in danger. The only way the caretaker can die is if he manages to get some poor idiot to agree to take over.
Enter Nathan Brazil, who has seen several universes come and go and is untold trillions of years old. He wasn't even a Markovian to begin with when he agreed, and the Well World made him into one, with all the capacity that comes with it (ie, full capability to conceptualize and control the growth of a new universe).
As a race on the universal level, I find these guys hard to beat. They effective controlled all aspects of their universe, and were unkillable by anything short of something capable of destroying said universe.