Well, I'll have to say Kanda on this one. He has the killer instinct, superior strength/defence and very strong will to survive.

If they're fighting to kill each other (no holding back) Kanda would definately win. Lenalee would be very fast and land MANY good attacks (which he'd likely heal), but Kanda only has to give one or two good attacks and she's dead. And even if Lenalee has pretty much won, and leaves him, thinking he's dead... terminator Kanda!!!

But if it's just training or something... I think Kanda would let her win... I'm not suggesting a KandaXLenalee thing... but he does make allowances for her (female thing or friend thing?)... I cant imagine Kanda letting Allen mediate with him, can you?