Love DS9! The most fascinating characters with deep ongoing storylines made it a must-watch for me.
But 2nd would definitely be TNG. Watching TNG feels like I'm visiting close friends or family. The crew was very tight-knit and I loved the weekly adventures, always running into something interesting. I appreciated that the writers felt no need to make it an action series and kept it close to the roots of TOS by having thought-provoking plots.