The Haitian is by far my favorite character, it's about time he got his own thread! As for his power(s?), it's/they're most probably just one power, given that Tim Kring stated that Peter and Sylar were the only Heroes to display more than one power during Season 1. The power negation most likely has something to do with the memory. But yeah, the Haitian's simply too badass, he's likely the most powerful hero there is at the moment, except for maybe Maury Parkman and Hiro in his own special way.
His ability to negate powers isn't absolute, true, however it's still a huge added bonus, for instance it grants him immunity to some pretty insane abilities (such as time manipulation or force persuade), and even when he can't stop the powers (which appears to be the case with radiation and flight) it's likely he can lessen the effect. But what I really find powerful about him isn't the power negate but the potency of his memory destruction ability. In It takes a Village he was able to mind wipe his entire freaking village, to the point where they couldn't even talk, and just became mindless zombies, and he did it in a heartbeat. The fact that it's a mental attack means that not many would have any sort of defence, and the sheer scale of the ability is immense.
I think it depends. In a close and direct confrontation between the two, the Haitian would realistically be able to block out his powers and mind wipe him. But given that Maury appears to be able to sense outside threats and attack from a distance, I'm thinking that The Haitian wouldn't be able to ever be in such a position in the first place.