This post shows different compression levels' effects.
Example file is Matomo tracking JS. See here. Its raw size is 64237 bytes.
| Compression level | Gzip | Brotli |
|---|---|---|
| 0 | 24965 | |
| 1 | 24290 | 24184 |
| 2 | 23499 | 22290 |
| 3 | 22878 | 22004 |
| 4 | 21611 | 21357 |
| 5 | 21021 | 20047 |
| 6 | 20873 | 19933 |
| 7 | 20850 | 19880 |
| 8 | 20847 | 19847 |
| 9 | 20847 | 19836 |
| 10 | 18527 | |
| 11 | 18189 |
So, it seems that level 6 is a good choice.