I would choose....
A SHOOT MATCH BETWEEN, SHANE DOUGLAS and SHAWN MICHAELS,
I can fap to this.
Personally, I want to see Lesnar vs someone new, but good in ring that can work with big guys. Cesaro and Daniel Bryan are the first two that come to mind. Cesaro has proved that he is not just good, but tremendous in ring. He has worked with plenty of big guys, so I see no trouble in letting him go with Lesnar with a well balanced fight between the two. It would give Cesaro a huge rub to be toe to toe with the beast, and they'll put on one hell of a match.
Daniel Bryan for the same reason, he works great in ring and they would put on one hell of a match. I say give Lesnar though more of a upper hand in the match since Daniel plays the underdog much better than Cesaro, and doesn't really show the strength that Cesaro does during a match.