I just watched "Trials and Tribble-Ations", "Flashback", and "In a Mirror, Darkly" for the first time recently, so this is a perfect time for me to be asked this question. I agree that "Trials and Tribble-Ations" is the best of these episodes by a wide margin and my favourite episode of DS9 overall (although I haven't seen seasons 6 and 7), but I've got to give a lot of praise to "In A Mirror, Darkly". I was amazed by how awesome it was.
The only other Enterprise episode I've seen is "Regeneration", and after I saw that, I was so disgusted that I said to myself, "screw this show, I'm never watching it again". I'm glad I changed my mind for the two-parter, though. Scott Bakula came across as awkward and trying a little too hard in the first part, but really improved in the second part where his villainy became incredibly entertaining and convincing. Having just seen "Mirror, Mirror" and "The Tholian Web" not long before this episode, I thought it brilliantly connected with the continuity of them while also standing alone as a great dramatic and occasionally comedic piece.
"Relics" was not bad at all. I thought the conversation between Scotty and Picard on the holodeck was incredibly moving, and the talk Scotty has with Geordi about being an engineer at the end was also quite touching. "Trials and Tribble-Ations" has a better story and more fun moments, but this one was more emotionally stimulating. I really felt for Scotty feeling alienated in this new time period and like I said, I found his talks with the crew members very sweet. His scene with Data was very cute and funny too.
As for "Flashback", while "In a Mirror, Darkly" changed my mind about "Enterprise", this episode only reinforced my negative perception of "Voyager". I've always found it to be a dull and plodding show, with uninspired characters, and I thought this episode proved that because even with George Takei around to inject a much-needed boost of charisma into the show, it was still a bore. The only "Voyager" episodes I've liked are the Q ones, and I think they only worked because of the guest stars, which allowed the episodes to succeed in spite of the writing and characters.