Mousasi/Rockhold hands down. They're both slick on the ground and have great, yet different stand-up. Luke's a southpaw kickboxer with good footwork, but he's not like Lyoto, is more in striking range which would make that more exciting.
Rockhold already beat both Kennedy and Jacare, and Jacare beat Kennedy, too. Rockhold/Kennedy wasn't close, but those other 2 fights were, though still clear. A Mousasi/Jacare rematch would be the most compelling since that was an upkick finish early in the fight.