What are some of your favorite would be revenges? One fighter getting revenge for another's loss, first ones that come to my mind are Tyson over Berbick, or the Klitschkos taking out those that troubled each other. My favorite would likely have to be Witherspoon Vs Gonzalez, an old veteran taking revenge for the bully's beating of a faded Renaldo Snipes (Who would've mopped the floor with Gonzalez if he wasn't old).
Not sure if it's my "favorite", but what came to mind immediately is Erik Morales posing to pictures standing over Fernando Velardez after knocking him out. Velardez got a DQ over Erik's brother, Diego - 1 year earlier after Erik's father stepped into the ring in the middle of a round, complaining about Velardez's fouls. There was some bad blood there and Erik was certainly happy to get that win back for the family.
Young SRR fought on the undercard of one of Armstrong's losses to Zivic, & swore that he would avenge his idol Armstrong, which he did about a year later in the same arena (but also ended up beating Armstrong too).