Kane would win. He is now being portrayed as a strong, indestructible monster. They are doing the same with Lesnar, but he seems to be on quite a losing streak here. Kane is the better big man and would pin Lesnar.
I'll never understand these threads. Wrestling is all a work, so the outcomes are predetermined. Therefore, asking who would win isn't really a question worth asking, as the winner is decided by the booking team, not by the wrestler's ability to beat another one.
If we're talking a proper fight, a shoot, then I have no idea.
Well, Kane was one of the first (if not the first) people to beat Lesnar back when he was in OVW. The only time they've come in any contact on WWE Tv was a few seconds in the last Royal Rumble.
If they ever do have a cross-brand promotional match, I'd rather it be Kane/Undertaker though.