Based on what I can remember, Kong seemed more impressive. He fought off 3 of those T-rex-like dinosaurs while protecting someone, IIRC, whereas the Indominus rex had trouble with one T-rex and a raptor (though the mosasaurus got the kill), from what I can recall. Admittedly, it's been some time since I watched either, so some specific details might be slipping my mind.
So, I would side with Kong here. Won't be easy though. Indominus will probably leave Kong with a good number of bloody wounds when all is said and done.
Kong in a fairly one-sided fight due to his size, fighting skill, and of course dexterity due to having hands and feet. Kong can grab, punch, kick, jump, and grapple, and does so intelligently and to great effect against multiple V-Rexes (I didn't know what they were called, thanks King) in King Kong 2005. I-Rex hasn't a chance here.