Imho what's the best compression software depends mainly from the user's goal... for backup or sharing data I would still use plain old zip (or tar+gz on Linux), without looking for the maximum compression or advanced features: the process just needs to be fast and the output to be readable from anyone.

If I'll need some more features and better compression, I'll go for Rar or 7-Zip; they are reasonably fast, they are great archiving format and are reasonably known by most users.

If I'll aim to pure compression, well, now the top performer is PAQ so I'll go to one of its GUI like:
KGBArchiver http://kgbarchiver.net/
PeaZip http://peazip.sourceforge.net/
PowerPaq http://sourceforge.net/projects/powerpaq