I think to find a good partner for M-Kangaskhan, you need some one who's a counteraction for his super effectives, plus great speed and attack with it to.
So far, fighting's super effective are Psychic, Flying, and Fairy. I don't want to waste your time using legendaries such as Lugia, so I've narrowed it down to 2:
Togekiss vs. Gardevoir
Between this, these are probably some great Pokemon to satisfy your team. The thing is, you already have a mega on your Team, and Gardevoir could mega as well. But, Togekiss excels in the necessary stats, better that Gardevoir (mostly because of his great defenses as well). So I think you should chose Togekiss as your M-Kangaskhan's partner.
I definitely recommend getting STAB powerful moves like Psychic and Dazzling Gleam - and Aura Sphere isn't bad either. Of course, to get these moves, you might have to train a Togepi or Togetic. But either way, it doesn't matter. This way you can give him another move or two that you think he would work great for him or the team as well.
In the end, if you don't decide to go with my decision - the best thing I think for you is a Balanced Pokemon that is great for counteracting the stronger foes.
Your Choice! Hope I helped! :)