In fiction............. Fiction.
We are talking not just comics and anime, but video games, books, tv shows, movies, etc.....
There are some people who are well diversed in arguing for some of these work of arts like myself, but not all of them.
I'll just name some univerese and you can pick from there.
1. Marvel
More than likely the top 5 will come from here. aka ToAA, Squirrel Girl, Beyonder, etc...
2. Dr Who
Time Lords op
3. Loney Toons
Bugs Bunny and Toon Force
4. DC
Because Batman. Ok fine, Presence, Specter, mr mxyzptlk, Great Evil Beast
5. Image Comics
More of the same thing DC and Marvel have like MOM and Solar Man of Atom
6. Tenchi Muyo
Any verse with a Omnipotent is OP.
7. Demonbane
Elder God Demonbane is 4 times the size of STTGL and just approaching a Universe, it blows up.
8. Archie Comic
Sonic fighting guys who can blow up multiverses by crushing them in their hands..
Beings who have to be killed on alternate planes of time, yeah op
9-29 Insert other works of fiction here
Saint Sayia, Slayers, Bastard!!, Star Wars, Darren Shan, Asuras Wrath, God of War, Digimon, Pokemon, Kirby, Devil May Cry, Xenogears, just throwing out names people have heard and will realize how hard this list really is...
30 DBZ
Because someone will mention they are are the strongest but they really aren't