#1 Drew McIntyre

Traditionally, Brock Lesnar's greatest matches have always come against smaller opponents that can bump around for him, but given how WWE see Drew McIntyre, it's only a matter of time until the two big men collide. While both men are absolute monsters, they're both very talented and skilled inside the ring, so not only would this be a top battle, it would feature two men who can work and tell great stories.
We've seen the likes of Roman Reigns and Seth Rollins having to overcome The Beast before they could break into the main event scene, and if WWE are serious about McIntyre, he could be next on Brock's list.
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Ultimately, the Scottish Psychopath is going to do huge things in WWE regardless, but if he can have a hard-fought bout with The Beast (akin to Reigns vs. Lesnar at 'Mania 31), it will serve him better in the long run.
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