Cain had Brock a bit earlier when the mystique was somewhat still there and outwrestled him.
Reem had him later, but controlled it in its short entirety and made Lesnar look incredibly scared throughout. 
HHH beat him pretty badly but i would say Cain
I'd say Cain. Brock was a soulless punching bag when he was in there with Overeem.
Overeem outclassed Brock very badly. I would say him. Brock was somewhat competitive with Cain.
MmanBearPig - HHH beat him pretty badly but i would say Cain
LOL but rumor was that Brock had completely shattered HHH's arm with a Kimura.
mir
The way Overeem finished that fight was beautiful. Those knees to the body followed by the kick to da Liva!
Cain simply had Brock spinning... out of control!
Looking back Brock wasn't the same after his illness, that doesn't mean he would have won either of the fights but i think he might have put up a better fight.
I think Overeem had less of a fight compared to the Cain fight.
BOMDC - I'd say Cain. Brock was a soulless punching bag when he was in there with Overeem.
This. 
The holes in his game were exposed. Did his illness affect him? Maybe, but the fact remains that he fought superior strikers and lost. This is coming from a huge Brock fan. He represented a guy who was very good at one skillset and I looked at him as a modern, less badass version of Mark Coleman.
Tank by gogoplata - The holes in his game were exposed. Did his illness affect him? Maybe, but the fact remains that he fought superior strikers and lost. This is coming from a huge Brock fan. He represented a guy who was very good at one skillset and I looked at him as a modern, less badass version of Mark Coleman.
Imagine Lesnar with head butts it'd be like getting a 20lbs turkey dropped on your face
Reem's win was definitely more dominant. Whether or not Brock was the same fighter as he was when he fought Cain is debatable though.
Does anybody really think Overeem wouldn't have done the same thing to ANY version of Brock?? Overeem's win was far more impressive, he made Brock look like he didn't belong in the same cage as him, and did it almost effortlessly and relaxed.
MachidasStraightLeft -
Reem's win was definitely more dominant. Whether or not Brock was the same fighter as he was when he fought Cain is debatable though.
I think this is the right post.
Cain is more impressive. The lesnar mystique was sky high after the carwin fight. He took a brutal beating and came back to win in that fight and seemed pretty unstoppable.
Reem dominated but it was common knowledge that Brock had been seriously sick the year prior and had been through a very invasive surgery. Reem targetted the weak spot and it was game over.
I think reem would have beaten Brock even if he was fully healthy but Cain stands out as the more impressive win given the mystique at the time.
Reem
MmanBearPig - HHH beat him pretty badly but i would say Cain
Goldberg clowned him at Wrestlemania 20.
That was awesome.
Cain... he was smaller and dominated brock... overeem did too but overeem was bigger and brock wasnt the same fighter.
Frank Mir's was the most impressive, Brock was green but at full strength. Post op Brock was a different person
Hmm.. Punches to the back of the head or a kick to the twice surgically repaired body.. Seems that is the question.
Honestly though, I think Cain did a better job against Brock than Overeem. After Cain got up following the takedown, he pressed Brock up against the fence and had him hurt. Overeem and Brock were even in the fight until Overeem hit him with the body kick.