I randomly decided to compare all the compression formats and methods and all the other technical stuff that 7-Zip supports compressing into, as well as the Windows Compressed Folder. Keep in mind that while 7-Zip doesn't support compressing into any other formats than the ones I compared with, it does support decompression of 15 formats "out of the box" and supports plugins to add support for more formats.
The files used in this test were the files that I distribute with my Stunting Grounds map, same as v0.31 but with minor changes to two of the text files.
Here's what each part of each line stands for in most of the lines: bytes, compression level, compression method, dictionary size, word size, and format. For the last one, solid means that all the files are compressed as a single string, which increases compression slightly but doesn't support modifying the files. The names in the list are, for the most part, the names that I used for the files during testing.
I couldn't go to the max word size for PPMd compression, which is 1536MB (which takes 1567MB to compress), because it said it couldn't allocate enough memory even though I had 2GB free of the 3GB that XP 32bit recognizes.
Here are my results, ordered from least compression to most compression, with the exception of the original size at top.

21,798,677 Original size of 10 files and 5 folders
21,810,176 store.tar
8,912,211 win_compressed_folder.zip
8,512,373 ultra_deflate_32k_258.zip
8,366,203 ultra_deflate64_64k_257.zip
7,180,180 ultra_bzip2_900k.zip
7,178,968 ultra_bzip2_900k.7z
7,059,709 ultra_lzma_64k_273.7z
5,310,828 ultra_ppmd_512m_32.7z
5,310,828 ultra_ppmd_768m_32.7z
5,310,828 ultra_ppmd_1024m_32.7z
5,229,221 ultra_ppmd_256m_32.7z
4,830,640 ultra_lzma_128m_8.7z
4,595,583 ultra_lzma_64m_273.7z
4,595,583 ultra_lzma_96m_273.7z
4,602,743 ultra_lzma_128m_96.7z
4,600,564 ultra_lzma_128m_128.7z
4,595,906 ultra_lzma_128m_256.7z
4,591,332 ultra_lzma_128m_273.7z
4,589,391 ultra_lzma_128m_273_solid.7z

Since there are a couple formats that only support single file compression, I decided to test that too. I used Stunting_Grounds.rfa.
The lines are in the same format as above but with the prefix sf (single file).

21,777,641 Original file size
21,779,456 sf_store.tar
8,901,909 sf_win_compressed_folder.zip
8,501,699 sf_ultra_deflate_32k_258.gz
8,501,798 sf_ultra_deflate_32k_258.zip
8,355,628 sf_ultra_deflate64_64k_257.zip
7,169,232 sf_ultra_bzip2_900k.zip
7,169,230 sf_ultra_bzip2_900k.7z
7,169,094 sf_ultra_bzip2_900k.bz2
5,302,623 sf_ultra_ppmd_1024m_32.7z
4,586,243 sf_ultra_lzma_128m_273.7z