There are alot of great instances but one of my favs is Durnholde Keep. Here you travel back in time by 7 years to the Old Hillsbrad Foothills. I even love the dragon flight down to the Caverns of Time. Although that LONG damn quest following that chick around is totally annoying hehehe.
My hubby loves the part after helping Thrall escape, he punches a guy in the face in order to get weapons. Everytime we go in that room my hubby says "wait for it, wait for it" POW! hehehe
You can do this instance at level 66 but, its no walk in the park at those levels. Lots of attacks on Thrall coming out of the Keep and its difficult to keep up with them at those levels. You need at least of couple of reasonbly geared 70s to take some of the stress off. Mages you can actually do some sheeping in this instance.
Following Thrall for the first time when he mounts up was very exciting and helping him to free Taretha's (although she never seemed captive to me) hehe. I love the feel of this instance and the history that goes with it.