The first seasons were much more diverse than the later ones, which is why I prefer them.
There is much more going on, more variety, more arcs, etc.
The Dominion War arc dominated the show and eventually became a bit repetitive. Sure, there are some amazing episodes in that long arc and it was a bold and exciting move for Trek to do a long war like that.
But Seasons 2 and 3 are easily my favorites with some amazing, imaginative episodes which are often very different from each other in tone and encompass many different corners of the Trek/DS9 universe.
The show became a little too homogenous and the war pushed aside many other great aspects of the show.
The show had a very fresh and original feel in the early seasons and I remember their episodes very distinctly, whereas, once the war takes over, I have trouble in my mind separating Season 4 from Season 5 and Season 6 from Season 7.
Season 1 is also vastly underrated. Here are some standouts from that season:
"Emissary". The second best Trek pilot ever after "The Cage".
"Duet" - easily in the top 3 DS9 eps and top 10 of all Trek.
"Vortex" - a vastly underrated, Odo-centric gem with some cool backstory about changelings.
"Captive Pursuit" - featuring one of the best one-off Trek guest stars ever.
"The Nagus" - First and easily the best Ferengi comedy episode (which, granted, isn't saying much, but it's a hoot.)
I also like many other episodes, even if they're not outstanding, because of how new and exciting and original the characters and setting of DS9 are. It's great to get to know them for the first time through solid (if not exceptional) episodes like "A Man Alone", "Babel", "Past Prologue" and "Dramatis Personae."
Granted, some episodes do suck out loud, like "If Wishes Were Horses", "The Storyteller" and the weirdly hokey "Move Along Home."