Good lord, I love Deep Space Nine. I can't really watch it while I'm doing other things, either, because I get all gripped. It's impossible to pick just one episode when you're loving on a particular character, because it's so arc-y. I guess my fallback episode(s) when I need a DS9 hit is the trilogy from the beginning of season 2 ("Homecoming"/"The Circle"/"The Siege"), because omgzzzz, that is the moment when I went YES, YES, THIS SHOW NEEDS TO BE THIS. And then it was. And it was good.
For the record, my current default-I-need-a-hit episodes for other shows:
Atlantis: "Hide & Seek"
Babylon 5: "Soul Mates"
TNG: "I, Borg"
Yeah, yeah, I know, I'm 90% sunshine and rainbows and snark. :-P