T&T because it alluded to the mystery of the Klingon foreheads, it put DS9 crew into TOS uniforms and it fitted into just such an iconic episode made all the more iconic by their visit.
Hated Relics because in my opinion they just sent Mr Scotty off like a doddery old man. Liked that they brought Scotty into the TNG timeframe so he could work his miracles again but they managed to fumble his character.
Flashback was cool in that we got to catch a glimpse of the Exclesior and see Sulu as Captain but all of the above criticisms of it are just too true.
In a Mirror Darkly was a great tip of the hat but more so to the MU - although they did go the whole hog and change even the show opening credits for the episode. That in itself is an homage.