You have three options.
1. At the beginning. The art and tone take time to evolve, but even some very early stories, like Mother's Love, can hit hard.
2. At the very peak. Grasscutter is one one the finest stories ever put to print. It is heavily based on Japanese mythology and has everything a good comic should have.
3. Anywhere. While there is some continuity in the comic, Usagi is easy to pick up and the reader will never feel lost, because everything you NEED to know will always be explained. If you want to be extremely selective, I suggest checking out Dragon's Bellow Conspiracy, Daisho, Traitors of the Earth or Mother of the Mountains.
Lasty, Usagi had a few runs (at different publishers), but they all tell the same story and each continues where the last one had ended (so if you want to read it in order, just follow the dates). There also were two cool, but short spin-offs, Space Usagi and Senso (Senso is best when read last). You can find everything on the internet (at the usual place for reading comics online), but if you really like it, then we suggest you consider supporting the official release.