Depends of your definition of "best". Martinez-Chavez was definitely the most DRAMATIC fight of the weekend because of the last round, and because it was the biggest fight of the weekend. But then again, the first 11 rounds were too one sided to call it a great fight./ In terms of sustained action, and competitiveness the answer is easily Beltran-Martinez. Maidana-Karass was a pretty good scrap as well, but for the most part, it was kind of one sided in Maidana's favor.
Of the ones I saw, the Maidana vs. Karass clearly. Just wish the ref had let it go longer. Shitty way to end a very competitive fight.