My psittacus was beautiful but he was a brute and roughed up most of my fish. He could hang with the large geo species like G. altifrons and G. winemilleri, and he did well with Saxatilis pikes, but he was a bit pushy to more peaceful stuff.
My vote is for Uaru. They may not be the flashiest species, but they're endearing, peaceful, and large-growing. Just be sure to feed them plenty of veggies and stay on top of your water changes as they're very sensitive to poor water quality.