I would use the Lead-208 as one isotope and try to fuse it with Calcium-48. I would choose these two isotopes since both isotopes are highly stable and are both doubly magic with the number of protons and neutrons in their shells. Also, Calcium-48 has been a key element in the creation of other elements. I believe fusing these two would create the most stable heavy element since both of them are already highly stable with doubly magic amounts of protons and neutrons.
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