I wouldn't say THE biggest fight, but definetly one of the biggest ones.
Brock clicks with casual fans almost better than Randy (and possibly even more) and lets face it, no casual fan wants to pay to see Randy get humiliated and destroyed by Fedor. Against Brock it would be an exciting back-and-forth match, but there's no chance of beating Fedor. I think that's what the UFC's plan was all along, to put on an exciting match with 2 big names instead of watching a rather unknown fighter (in the eyes of the casual fan of course) in Fedor versus Randy. It would make the UFC look bad, Randy look bad, and it wouldn't draw as much.
Jack Johnson: Boxing's greatest Heavyweight champion.