MUI Goku + FP Broly + Golden Freeza + Beerus + LB Jiren + Hit + Moro. They all fuse into a single warrior with the strengths of all but weaknesses of none.
I mean, a fusion of SSB Goku + SSB Vegeta alone was above FP Broly -- and Broly was heavily implied to be > Beerus. So THIS fusion would be inconceivably powerful... Thing is, we have absolutely NO clue what Whis' upper-limits are, aside from the fact that he's below Daishinkan.
...The possibilities you can think of for this theoretical fusion are insane, though. Like, what would MUI + time-skip be capable of, for starters..?
On another note, I've always wondered if Whis could do this: (please log in to view the image)
There is no objective way to assign a hard limit to Whis. He is stronger than Beerus to such an extent that he can casually one shot him with a chop, but below the top tiers. That's a huge gap.