What do you define as a "filler" episode? Some of DS9's finest moments were standalone stories, and often they were money-saving shows too.
Alright, a couple of episodes I consider to be filler, and not particularly good filler:
Looking for par'Mach in All the Wrong Places
Doctor Bashir, I Presume?
A Simple Investigation
I haven't finished the season yet so I'm sure there's more.
The Pah'Wraiths are present in The Assignment and it fits into the character-building episodes of the 5th season. Doctor Bashir, I Presume gives Alexander Sidding something to do with his character and soon it will tie into the main storyline. A Simple Investigation is Odo's first love affair as a human. It tests the limits of the order of his universe.
You just named 4 character-building episodes I wouldn't do without.