The original series or next generation depending on your taste. Although be warned that TNG’s first season is rough, so if you’re not committed to watching everything, maybe pick a few episodes to try first.

Personally I watched through the entirety of TNG and then moved on to DS9, and I really enjoyed both shows. DS9 is also a direct follow up to TNG in a few ways so they make a good pair. I wouldn’t recommend watching DS9 first though since it’s so different from “normal” star trek.