HCEnc is not a Libavcodec based encoder. It was written from scratch.
Quenc, Freenc, and Nuenc are.
I Like HCEnc. It's slower than Quenc but has better quality IMHO. The author wrote it from scratch using Fortran. It's pretty impressive considering that he did it all by himself. Hopefully the future versions will have better performance.
Quenc isn't bad either. It's good enough for most jobs and is faster than HCEnc. I don't know too much about Freenc or Nuenc since I've never used them. But I do agree with Fritzi93 that they are probably no different than Quenc since they are also based on the Libavcodec.
All of the encoders above can also beat CCE quality with the right tweaks.
If you would like to know more about them I'd suggest going to the Doom9 forums. They can give you better explanations.
I hope this helps.