Cell absorbing Goku tho.
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Imagine if that shit had been permanent. I think Cell would've done some irreparable damage to the GT universe assuming he'd have Goku's SSJ4 level of power in addition to his own, or been able to tap into that. That kind of Cell would curbstomp any Buu.
Even if Cell with GT Goku absorbed isn't accounted/allowed for this thread though, what if he did a Solar Flare and then proceeded to absorb Fit Buu?
Either of those options is literally the only way he could win here.
edit: Then again, Buu could possibly spit himself back out of Cell like Goku did on account of being stronger, so that might not work even

, but seems worth bringing up nonetheless for the sake of arguing.