Ok, so this one is a bit of a hard call. Season Six is easily the most serialized season so far. The first six episodes are basically one long story. I only ever do a list of five essentials so I'm just going to suggest one of those episodes and assume you can continue with the rest of the essentials from there.
Episode 130- Sacrifice Of Angels
-This is the episode where Sisko and the crew make their big push to get back on the station. Lots of plot stuff happens here--very dramatic and important stuff.
Episode 134- The Magnificent Ferengi
-Watch this one for some SERIOUS Ferengi hijinks. I mean, for real. Seven Ferengis and Iggy Pop. You absolutely cannot go wrong. Also, if you're going to watch the rest of these, you're really going to be glad you had a Ferengi romp in this list. Trust me.
Episode 137- Far Beyond The Stars
-In spite of my qualms with this episode I still think this is a beautifully done, beautifully written, and expertly costumed and designed episode. The idea to plunk all our DS9 friends into the 1950's is bizarre and just the right kind of quirky for me. Also: it's totally iconic. You can't not watch this one.
Episode 149- The Sound Of Her Voice
-Ok, this may at first seem like a strange addition. There's literally zero attempt at furthering the season's arc. The plot rests solely on the Defiant crew reaching a marooned captain before she runs out of oxygen on a desolate, L-class planet. On the way to her, each member of the crew takes turns talking to her so she isn't alone in what could be her final hours.
I selected this one because the crew's confessions to Captain Lisa Cusack are revealing and heartbreaking and made even more poignant in the very next episode--the season finale.
Episode 150- Tears Of The Prophets
-The Season Finale. Lots of feels. Lots of plot. If you don't watch this one you're going to be REALLY confused in the next season.
Episode 136- Who Mourns For Morn
Confession: I only JUST (like last week) realized that Morn is an obvious tribute to Cheers' Norm complete with anagrammed name. I feel like a real petaQ.
Anyway, this episode is all about Morn cacking and Quark having to chase down the secrets Morn left behind. If you're at all interested in Quark's most dedicated patron, you need to watch this one.
Episode 138- One Little Ship
Here's an episode where half the DS9 crew shrinks down to the size of a tennis ball and pilots a teensy runabout around the Defiant. As ridiculous as this sounds, this one is actually surprisingly great.