Both are the monsters who can't be killed, Buu can regenerate from pieces same as he fought Vegeta and Doomsday only can be KO'ed same as Deadpool

Round 1: Standard Doomsday vs Fat Buu

Round 2: Standard Doomsday vs Kid Buu

Round 3: HP Doomsday vs Super Buu

Round 4: HP Doomsday vs Fat Buu

Round 5: Bloodblusted HP Doomsday vs Super Buu(Gohan Absorbed)

No morals, BFR are allowed, KO only because both cannot be killed 😄. Takes a place in the place where Gohan fights Super Buu

Who wins?