What closed attraction do you most regret never experiencing?

Deep Earth Exploration at MGM Grand Adventures. And bring a camcorder with me.

Oh, heavens, which ride I regret not bringing a video camera on is a whole 'nother question and much longer list!

The Trip: Nights in White Satin at Hard Rock Park

For anything, Loggers Run @CGA, skipped it for no reason even though it was a walk-on and boiling hot (that one still stings). For coasters, Little Dipper at Scandia, as it's closing extremely soon and I don't think imma get out there before it dies

The Star Trek Experience in Las Vegas. I have never been to Vegas and wasn’t a Trekkie when it was open- now that I am, it sucks I never got to experience a dedicated themed spot to 80s/90s Trek. Also, I wish there was a way to recover the taped session from the Star Trek attraction at Universal from the early 90s. We had a tape of my family doing it when I was 2, but the tape mysteriously vanished and now like the Road Warrior lives on only in my memory.

The ExtraTERRORestrial Alien Encounter at Magic Kingdom, it was hands down the best attraction at WDW.
Jaws at Universal Studios Orlando. I only road it once at night and it was amazing, I'd like to do that again too.

Twister...Ride it Out @ USO - from the videos and write ups, I think it was certainly my kind of attraction.

If I had a time machine, other than revisiting lost friends and family, I'd be off for a trip through time to visit the Great World's Fairs. I'd start with 1851 in London and hit them all up to the most recent. I am especially interested in the Brussels '58, San Diego '15, New York '39 & '64/65 and Seattle '62 I don't collect much in the realm of Theme Park souvenirs, but I have a HUGE collection of items from the Worlds Fairs as far back as the 1904 Fair in St. Louis.

I only visited Disneyland once while the SkyWay was in operation. I'd ridden the version at Magic Kingdom dozens of times, but I never got the chance to glide through The Matterhorn. I regret not having that experience.

Top 5:

-Rocket Rods, because it was at my home park yet I never made an effort to actually try it out.
-Big Bad Wolf, because it was widely considered the best Arrow suspended and I missed it by just one year.
-Jaws, because from what I've seen and heard it seems like it was a really good attraction.
-Blackpool Pleasure Beach's Wild Mouse, because it was such a rare ride type and reportedly the best remaining of its kind.
-Son of Beast, just to say I'd ridden the largest wooden coaster ever built.

Hard Rock/Freestyle Music park is an honorable mention because while I had plans to go there in 2010, nothing in particular at that park really appealed to me.

The 'Chicago Loop' coaster at the Old Chicago theme park.

Holy Cow! How could I have forgotten "The World of Sid and Marty Krofft?!" I was only a little kid during its short life. My family was going to go during the Thanksgiving break, but it closed on November 10th. I really miss not getting to experience those wack-a-doodle attractions.

Disney : Adventure through Inner Space
Meli park (now converted into Plopsaland) : Apirama (8 min darkride 1979, actually I've been in there, but I was young then, would like to see it "now")
Hard Rock Park, or rather "Sally inc..." : Nights in white satin - The trip. (It was a work of art, rather then a conventional dark ride)
Frontierland (Morecambe) : Noah's Ark (there is only one remaing Noa's Ark in the world, now... but I actually experienced it in Morecambe before the park was demolished)

@Rob McCullouch
The Brussels '58 Expo, was indeed the first in the row after WW2 and an explosion of enthousiasm & belief in progress.
Parts of the Expo had a longer life, and the Atomium still is there, shining bright as never before (after major renovation works, years ago.)
Theme park fans should visit it, exploring several of the flashy spheres from the inside by means of escalators, stairs and the fastest elevator in the world, at least in the opening year 1958. Still now, the elevator "crew member" is narrating the building story life (In 4 languages :-) )

There was a brief time in the 70’s or 80’s where USH had the classic cylons from Battlestar Gallactica as part of the “backlot tram tour.” As a kid I would see the commercials and beg to go. I’m sure it was complete cheese, but I never got to experience it.

And many of you reference the classic EPCOT attractions - I never got to experience Horizons, World of Motion, or the original Imagination.

My big one here is Big Bad Wolf. That and Loch Ness Monster are the only roller coasters I ever chickened out of riding.

I would have liked to see Hard Rock Park, but I don't think I would have appreciated it nearly as much as I do now.

I am one of the few who was able to make it to MGM Grand Adventures while it was open at full capacity. I shake my head because while there were some really great things going on at that park like the Three Stooges show, for the most part, it was a lot of "not quite enough". The roller coasters were not very tall, the themed rides were extremely cheesy and there just wasn't enough to do for the price they were charging.

@Herwig Delvaux - I have visited the Atomium twice. I lived in Brussels for a year in 1986 with my Aunt who was a teacher at the American School. Her house was in Wezembeek-Oppem. I love Brussels and feel like it is my European home town. I went to the park site while I lived there, but the Atomium was in rough shape as I recall at the time. I was back in 2009 and visited the park site again. I was BLOWN AWAY by the renovation done to the Atomium. It was fantastic. I was so happy to see it all shiny and looking new again. The tour inside was fantastic as well.

The only one for me is a very old one: 20,000 Leagues. It kind of amazes me that the studio seems to have forgotten this as a movie as well. No high definition release of arguably Walt's proudest live action film, which stands up pretty well. There is a thread about Universal bringing back defunct attractions at their new park, but Disney desperately needs an Orlando fifth gate, and this would be a great one for that park, along with Mystic Manor next to it.

I'd say Jaws, although my wife seems to think I missed out most with Back to the Future. Honorable mention goes to Osborne Lights and the old school Shamu shows.

I'm going to restrict myself to rides at parks I visited and was alive to see. In that case, I'd say "Back to the Future" and "Kongfrontation" at Universal Studios Florida and "Alien Encounter" at Magic Kingdom.

Rocket Rods just to see if it was that bad or not.

Alien Encounter at Magic Kingdom. Holding out hope that Disney will reincorporate a ride like this now that they are acquiring the rights to the Alien franchise.