Assuming it HAS to be a Zoan? Marco's Phoenix Fruit would round him out and basically provide a full package.
Darkness Fruit lets him negate Devil Fruit abilities, largely removing the need for Haki. Of course, there's the lack of dodging, and the only moderate combat value of his powers (as they've been shown so far).
Earthquake Fruit lets him deal with powerful opponents and large numbers. It's primary fault would normally be that it's so powerful that you wouldn't necessarily have a strong Haki, due to over relying on it, but the Darkness Fruit obviates most of that.
Phoenix Fruit lets him get around the 'I can't dodge' aspect of the Darkness Fruit, while also providing a nice, non-Haki based layer of protection. Armament Haki lets you strike the 'real body,' and unlike Logia or Paramecia, Phoenix Fruit's body is always real, it just heals crazy-fast.
The three combined would provide a solid set of powers that would be difficult to get around… You'd have to use Seastone, or somehow outpace his regeneration.
As for what he'll probably actually take? Something big and powerful. He's into overwhelming force, more than being a potent all-around type. So... maybe a rhino? Or an ape? An Ancient Zoan, perhaps? OH! Human Fruit Model: Giant.