Tough call. Going with MAB vs Morales. Two Mexicans fighting for the right to call themselves the best fighter from Mexico at that time who hated each other made for higher stakes.
Both very good trilogies. But Morales-Barrera was better, better fighters, better skills, closer fights and a real grudge match between two proud fighters who really disliked each other,it had everything(other than a KO). Plus Morales-Barrera I is the best fight in the history of boxing IMO.
Regardless of what HBO revisionist history would have you believe, Gatti and Ward only ever had one great fight, which was the first. The other two fights were one-sided affairs with Gatti incorporating a lot more lateral movement, being more economical with his shots and doing a lot less trading in the pocket, which rendered Ward hopeless. Morales and Barrera had two great fights with some of the best, most vicious exchanges that you'll find in boxing. The best rivalry/feud in boxing over the last twenty odd years, easily.