To be fair, it's hard to point out a single application as the best .nrg to .iso converter. There are several good ones and they all pretty perform the same when it comes to converting the file. Some are free, some aren't.
One application I've personally used to convert image files between different formats though is UltraISO. Converting image files between different formats is not its main feature but it is one of many available features.
To convert an NRG file to ISO in UltraISO follow these steps:
1. Go to Tools -> Convert in the upper menu bar. A new window will open.
2. In the "Input Image Filename(s):" field you should select the NRG file you want to convert.
3. In the "Output Directory" field, select...