what is the best file compression?

7-zip and RAR formats give best compression. bz2 also makes very small files.

A few months ago I experimented with JPG files using each format available for use with IZArc and found that BZA format resulted in the best compression most of the time. But if I'm sending a file outbound I use ZIP since that is a most common format most "ZIP" programs are capable of handling.

7-zip and RAR formats give best compression. bz2 also makes very small files.

I've found that 7-zip seems to outperform RAR consitantly. I haven't done very extensive testing though.

As for compatibility, I think RAR is a far more common format than 7-zip, at least at this point. If you are looking for universal compatibility, ZIP is probably your best bet. Windows XP can even support that right out of the box.

i would use 7-zip for sfx archives (although there's a lot to improve on 7-zip sfx modules) and rar for common compression

I would say uha compression. Although it is slow, it works very well. There is a nice GUI for it here: http://www.klaimsoft.com/winuha/

I didn't have a chance to play with winuha before today. It works better for me than IZarc set for BZ2. Here be the results. UHA was using ALZ:2-mode (its default setting). Ran a bit slower and had the expected difficulties with files already highly compressed. Installs with a right click menu which is a MUST in my book. For local use I'm going to go with winuha.

D:\Program Files original size 299,865,789 bytes (from less used Dual Boot drive)

ALZ:2-mode was 4.98% smaller than BZ2

80 JPG files original size 31,636,192 bytes

ALZ:2-mode was 0.40% smaller than BZ2

682 files a variety of CAT, CAB, ZIP, INF, EXE, TXT

ALZ:2-mode was 31.45% smaller than BZ2

IE6 SP1 Update Rollup MS (full installation EXE)

ALZ:2-mode was 0.93% smaller than BZ2

UNOFFICIAL Win98 SE SP 1.5 (EXE)

ALZ:2-mode was 0.41% smaller than BZ2

USB Host to Host Bridge PC2PC (DLL, INF, SYS, BIN, DAT, CAB)

ALZ:2-mode was 17.25% smaller than BZ2

NVidia (three different versions in EXE format all contained in subfolders)

ALZ:2-mode was 1.21% smaller than BZ2

2,463 files in 368 folders in Unzipped Folder (where I unzip programs that have no setup)

ALZ:2-mode was 16.12% smaller than BZ2