Budget?
System, OS, Hardware?
DAW?

Well I own and use Izotope RX and I'm pleased with it. My daughter used it at school and in a studio where she was an intern and she knew how to use it and liked it. I can't say that I did a comprehensive audition of all competitors. If you can sample HVAC noise or guitar amp hum it does a nice job of eliminating noises like that. That's what I've used it for so far, and it's done the job well and pretty transparently. I really can't say how it works on more complicated more intermittent noise.

How about 2 options
A)unlimited budget
and
B)under $500

I'm on Mac and using Adobe Audition currently, though I've been told Izotope offers better NR

Well, I'm partial to Bias Soundsoap Pro but I think that's just a matter of familiarity. $300 Waves has Z-Noise. $200. Izotope RX is Way more robust that the Waves Z-Noise.

Someone here mentioned Cedar Systems a long while back but those cost into the thousands of dollars as they have specialized hardware for specific tasks.

Izotpe Rx hell yeah