Compression depends on the type of data being compressed. it is possible to compress a 3gb file into 50kb.. that is if the file contains nothing but a 3 gb allocation without data..
I think there are old threads regarding these 50kbto3gb files. they are called bombers i think..
And most of text data..like a .txt .doc file can be compressed to high degrees as they contain repitition of similar characters..
I prefer .rar to .zip as the former is wide spread in distribution. You can see mostly .rar files in all distribtution sites..
in winrar, there is an option to compress to a maximum extent. it just takes hell a lot of time to compress some more..
7-Zip and Uharc are also very good. especially uharc has high compression ratios. this may be due to high compression algorithms.. But ..you cant compress certain data beyond its normal extent inspite you use better compressors..
Hope it helps