I really hope Brock wins... He is the best part of WWE atm. I would love to see him keep the belt till WM 32 where some new face of the franchise takes him down at last..
I love Daniel Bryan, but I don't see how he could be a realistic winner vs lesnar in storyline given Brock is the biggest "unstoppable force" in wwe history. Give him a good match vs Lesnar in the one of the PPV after mania, ditto guys like Orton and Cena. Have Rollins cash in, only to get squashed after Lesnar hulks up. Brock beats them all and become more heelish so whoever defeats him would get insanely over next year.
Roman Reigns could even be that guy in an awesome redemption story if he can improve his charisma/mic skills and get over before then.