My personal favourite is Hearns/LeonardII.

I think Zale/GrazianoII was probably the best.

Others modern superb rematches that come to mind include Bowe/HolyfieldII; Liles/LittlesII; Rigby/AyersII (not quite as good as the first fight though); MAB/Morales; Ward/GattiII and Robinson/GattiII;