Pac beating Hatton and then Cotto beats even that for me, seeing the size difference and how dominant both wins were. A former lineal FW champion beating the crap out of a, prime, future lineal MW champion is just mind blowing.
Some goods ones are: McLarnin's wins over Canzoneri and Ambers Conn's wins over Yarosz and Corbett III Moore's wins over Bivins and Johnson
Probably these two the more I think about it. Just a former lineal FW champion beating a current lineal LW champion has never happened as far as I know, and in that manner as well. Then following that up with blowing away one of the top WWs, who would go on to hold a title at JMW and then become lineal at MW... Has for sure never happened before, even if you take away the back to back requirement, and I don't expect it to during my life time. It's a singular feat.