I'm really fond of this season because it's around this time I started becoming a Trek fan and one of the last where I didn't already know ahead of time what was going to happen. I really like the universe at this point, where things are still being setup and it's still mostly just about the area around Bajor.
I voted for "Crossover" because I am a total sucker for alternate universe episodes but I also love where that stuff is still new in a TV shows life and feels weird and off and not routine (ie TNG "Yesterday's Enterprise", "Time Squared", SG1 "There But for the Grace of God go I", Buffy "The Wish", Sliders "Invasion"). I think there's a lot of fun ideas used like being back on a version of Terok Nor, the way the cast are used, and just being back in the Mirror Universe, which I think is an interesting use of that storyline rather than just being a kinda "Mirror Mirror" 2.0. There's nothing wrong with doing that other story, what I assume Diane Duane may have done with Dark Mirror, I just think it's really interesting to see that what happens after one of Kirk's adventures may not be the happy ending we assumed.
One of my favourite background details is the way Mirror-Quark doesn't know what latinum is, implying that Ferengi society is much different, and it's interesting to think that back then this was one of the first major uses of Klingons on DS9, as they were seen more as TNG's aliens and it was an interesting novelty to me.
I like "The Jem Hadar" a lot as well and have probably watched it more times. There's so much to enjoy in it but that moment when the Jem Hadar walked through the forcefield always stood out to me as suddenly revealing how outmatched Starfleet was and how dangerous these guys were.