Meh, after GT I was glad they didn't do another series (especially with them forcing yet more SS Transformations on us). If you follow GT, the most powerful being EVER in DB history is Cell after he absorbs Goku for like 2 minutes. His power far exceeds anyone elses. If we're following actual canon, then the most powerful of all was Vegito, by far.
And It's Time For...
Today's Moment of the Day - Galick Gun vs. Kamehameha
[YOUTUBE]jvj7E2JHJWs[/YOUTUBE]
They say this in the wiki but it isnt accurate. Cell may have absorbed him in hell but in the end of GT when the two SS4's fuse they become more powerful than anyone.
Unless of course you mean only singular being. But in either case who cares, non-canon.