#2 John Cena
Back in 2012, Brock Lesnar threw a backstage tantrum when John Cena no-sold his beating following their Extreme Rules match. Cena was supposed to be carried out on a stretcher, but he got up and delivered a promo to close out the pay-per-view. It was clear as day that Brock Lesnar wasn't a fan of Cena on that night. Reports started coming in soon after, stating that this real-life feud went way back to Lesnar's first stint in WWE.
Stories from behind the scenes say that Lesnar was not a fan of a young, up and coming Cena. One source actually stated, “Brock absolutely positively hated and detested John Cena!”
Lesnar reportedly bad-mouthed Cena to Vince McMahon many times, especially anytime Cena was doing something being perceived as positive.
Brock Lesnar wasn't a fan of Cena when both were mainstays on SmackDown
Brock Lesnar and John Cena worked on WWE SmackDown, back in 2002-04. Immediately after Lesnar's WWE title victory over Kurt Angle at WrestleMania 19, he kicked off a feud with Cena, which led to a match at Backlash 2003. Cena lost in the end, and it looked like Lesnar wasn't too cooperative during the bout. This was it in terms of their rivalry over the WWE title. They feuded for a short while on the road to Survivor Series 2003, where Cena was a part of Team Angle against Brock Lesnar's band of giants.