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

yeah i like 7zip because it does .gz and .tar and ..bz2 etc.... so its good for web developers etc... as many opensource projects use those formats

The best is none. You are trading use time for space.

True - Very true

but its helpful for downloading. A zipped file can take a fraction of the time to download.

I still prefer winrar because WinRAR can create multiple-volume archives (popular with file sharers) and use 128-bit AES encryption to password-protect them. Like most of its competition, it also has an option to hide file names inside an encrypted archive: someone without the password won't even see what files you've archived, let alone view their contents.

Try WinZip I am using it for some time and I am happy with it.

WinRAR is still kicking butt in the data compression world.
It's fast and very well-rounded. If you really want to squeeze out as much as you can, I'd install WinZip, WinRAR, and 7Zip then compress your data once with each one and see what one did the job the best. Sometimes the type of data dictates what program will work the best.

there is a free program called 7zip, it's good too

Just saw this thread and thought I'd also suggest FILEminimizer Office - it compresses all MS Office files up to 98%. The cool thing about this is that it compresses the actual file, so there's nothing to extract. If anyone's interested, here's a link to download a free trial: http://www.filecart.com/details/4263...zer_Office.php