I'd say, conservatively, it counts as entry-level high end. Indeed the panel type is of pro quality, and it certainly can be used for professional graphics work after calibration.
The best "top-notch" monitors come in the 2000 and up range from companies like EIZO. Of course, only the most demmanding professionals in graphics or production work would require one of those. For most people, you included, the iMac's screen could be regarded as top-notch.