Wow that's a great question. It's hard for me to narrow it down to just one.
I miss EPCOT Center. I miss the boring charm, the edutainment, the unrealistic future. I miss rides like Horizons, World of Motion, the Living Seas, MAELSTROM, Body Wars, Cranium Command. I miss Tapestry of Nations. EPCOT Center just had this charm to it that I can't relate put into words. It was just a feeling.
At MK, I miss Snow Whites Scary Adventures, 20k Leagues Under the Sea, Delta Dreamflight, Mr. Toad's Wild Ride, the Skyway, the old voice on the PeopleMover.
At DHS, I already miss the Osborne Lights, Streets of America, and the CM doing the narration during TGMR.
To quote an earlier poster: "If you are thinking of individual nostalgia then it is whatever was there on your first few visits that you really enjoyed." This statement holds very true. I am sure my kids will feel nostalgic about other rides in comparison to what I feel nostalgic about.