My favorite theme park attraction of all time was Universal Studio's Back to the Future: The Ride. It was absolutely flawless in execution and felt like an essential part of the Back to the Future movie trilogy in a way that other movie tie-ins can only dream of. Alas, it was gutted so as to install a ride for The Simpsons which is only half as good... so if you never got to ride it, you're out of luck.

Many people find it surprising that my favorite didn't come out of Disney, since they are the company that popularized the entire concept of a theme park. No, they weren't first, but they came along and did it bigger and better than anybody else ever had... partly in thanks to having such an established brand with a multitude of characters and properties that translated perfectly into attractions.

That being said, I decided to rank my favorites from both Disneyland and Walt Disney World into a single list. Partly because I want to make sure I don't miss an attraction for my time here... but mostly because I don't feel like working tonight...

1. Star Tours: The Adventures Continue (DL WDW)
   Proving that Disney was the right home for the Star Wars universe all along, this space flight simulation attraction in a moving theater got a serious upgrade when they randomized segments so it's practically never the same ride twice and made the whole thing in 3D. Epic in scope, it's a great Disney experience on either coast. Though the queue is far better at Disney World, the attraction itself is the same.
2. Pirates of the Caribbean (DL WDW)
   If there's a ride that sets the tone for everything that a Disney attraction can be, it's this one. The Disneyland version is vast, with an abundance of signature moments that build a complete "world" for you to inhabit. Even with the Johnny Depp upgrades, the ride still feels a bit dated, but it's such a great experience that I just don't care.
3. Soarin' (DL WDW)
   Originally called Soarin' Over California this immersive theater experience (complete with smells!) is so breathtakingly awesome that you never tire of it. The attraction was recently changed to Soarin' Around the World featuring world landmarks in a very cool way, but I kinda miss the California version.
4. Expedition Everest (WDW)
   Disney's themed roller coasters are always fun, but reached its peak (so to speak) when Expedition Everest. The fact that they added a new level of thrills by having a portion of the ride run backwards is just the icing on the cake. The Tibetan theming throughout the ride is immersive and wonderful.
5. Big Thunder Mountain Railroad (DL WDW)
   The roller coaster itself is nothing groundbreaking... it's the way that it's themed that makes this ride so darn compelling. Old West elements tied together with a runaway train on a fun track go a long way towards creating a memorable attraction.
6. Splash Mountain (DL WDW)
   A flume ride to end all flume rides, Splash Mountain integrates Br'er Rabbit and company into a terrific story that sweeps you up as it sweeps you along. The plunge at the end has real weight to it, making this a heavy hitter in the thrill ride department. The version at Disney World has side-by-side seating that's a lot more comfortable than the Disneyland version.
7. Hyperspace Mountain (DL) — Space Mountain (WDW)
   Having a roller coaster dark ride was always a fantastic concept, making Space Mountain an immediate hit. But then they added a Star Wars overlay and re-christened it Hyperspace Mountain which elevated the ride to a whole new level. Unfortunately, that's something they don't offer at the Disney World version, which would probably drop that version down on my list a few notches.
8. The Twilight Zone Tower of Terror (DL WDW)
   Leave it to Disney to make a tower-drop ride interesting. By building a creepy hotel around it and adding The Twilight Zone theming, Tower of Terror is one of those attractions that uses story to make the mundane something special. The Disneyland version has the drop-car leaving the elevator shaft and wandering out into the hotel, which makes it a much better ride than the static Disney World version. Unfortunately or fortunately, depending on how it turns out, the Disneyland version is dropping The Twilight Zone in favor of a Guardians of the Galaxy: Breakout ride. Could be a disaster. Could be just the ticket to rejuvenating an aging attraction.
9. Haunted Mansion (DL WDW)
   This timeless attraction is low on thrills, but the painstaking attention to detail and creepy atmosphere makes it a far better "haunted house" than it has a right to be. The original at Disneyland is probably my favorite... even though the extended queue at Disney World adds more cleverness to an already clever ride.
10. Mission: SPACE (WDW)
    Disney took NASA's High-G-Force Centrifuge and turned it into a nifty simulation of what astronaut training might be like for a mission to Mars. The result is surprisingly good, though not for everybody. If being accelerated to 2.5Gs is too intense for you, there is a "non-spinning" option that you can ride, though it kinda defeats the purpose of the attraction.
11. Toy Story Midway Mania! (DL WDW)
    Probably the most refreshing concept for a Disney attraction in ages, you board a moving car that takes you from one 3D "shooting gallery" to another, where you hit targets by pulling on a string attached to a "blaster" at the front of your car. It's cute. It's fun. And it's themed flawlessly for the Paradise Pier midway at Disneyland's California Adventure. Since Disney World doesn't have a midway, they have you taking a trip into Andy's room (from Toy Story) which is almost as cool.
12. Ellen's Energy Adventure (WDW)
    It's amazing to me how well this attraction holds up after 20 years. It's part comedy movie, part educational film, part ride, part diorama... and all entertaining. And, in addition to Ellen DeGeneres, you also get Bill Nye The Science Guy, Jamie Lee Curtis, and Alex Trebek with Jeopardy. Every time I ride it, I'm afraid that it will be replaced before I return to Epcot.
13. Radiator Springs Racers (DL) — Test Track (WDW)
    Disney took their Test Track technology (boarding a six-seater car to drive through automotive endurance tests, then a high-speed run on a test track) to a higher level when they built an entirely new addition to California Adventure around the concept of Pixar's Cars. In addition to a lot of huge animatronic characters from the film, you ultimately get to "race" against another car in a (relatively) high-speed drive through the Cadillac Mountain Range from the movie.
14. California Screamin' (DL) — Rock n’ Roller Coaster Starring Aerosmith (WDW)
    Two really good rollercoasters. It's as simple as that.
15. Indiana Jones Adventure: Temple of the Forbidden Eye (DL) — Dinosaur (WDW)
    While two very different concepts, both of these attractions feature a ride in "Enhanced Motion Vehicles" over a bunch of simulated hazards. You'd think that the Indiana Jones theming would make the Disneyland attraction a far better experience, but that's not really the case. In fact, before Dinosaur was re-themed to be a crappy movie tie-in, it was called Countdown to Extinction and actually a better ride. Now? Not so much, and I'd definitely put Temple of the Forbidden Eye first.
16. Kilimanjaro Safaris (WDW)
    Until I actually went on an African safari, I thought that Kilimanjaro Safaris was undoubtedly a highly abstract idea of what a real safari might be like. I was wrong. It's shocking just how faithful Disney was in trying to recreate the experience. The massive potholes in the roads. The sounds. The smells. The animals (when they're out)... this is as close as you can get without stepping foot in Africa. Yes, it's been Disneyfied, but mostly in a good way. As an aside... if you plan in advance and have money to burn, I highly... highly... recommend booking the Wild Africa Trek excursion (which I talk about in detail here). It is a fantastic experience that's worth every penny, and one of the best Disney attractions you'll find.
17. Grizzly River Run (DL) — Kali River Rapids (WDW)
    While essentially the same concept (you board a giant circular raft and go whirling through a water track), Disney World's Kali Rive Rapids is a pretty lame experience, built around the story of illegal logging and forest conservation. It also seems like a tamer ride. Disneyland's Grizzly River Run, on the other hand, took the idea to a better, if not slightly wetter, more extreme level... trying to simulate whitewater rafting through a terrific forest setting.
18. Buzz Lightyear Astro Blasters (DL) — Buzz Lightyear's Space Ranger Spin (WDW)
    Essentially a shooting gallery combined with a moving car dark ride, the Buzz Lightyear attractions in both parks were one-upped by Toy Story Midway Mania, and feels dated by comparison (despite being quite new). Even so, it's a fun ride that's worth your valuable time.
19. Jungle Cruise (DL WDW)
    This badly-dated attraction is in desperate need of a major upgrade, but will probably never get one. The best they can do is have the boat captains add jokes and bad puns to their tour dialogue, which is fun the first time you hear it, but gets really old, really fast. Still, it's a pleasant diversion from the rest of the park, and still a great concept.
20. Matterhorn Bobsleds (DL)
    It's not that I dislike The Matterhorn, it's just that it's been so massively eclipsed with Expedition Everest in every way that the original feels more like a toy than an attraction. Still, if you're visiting Disneyland and don't mind being crammed in tiny cars zipping around a track, this is still a pretty great ride. When it's open. Which it isn't a lot of the time.

Time will tell if this list changes after the new Pandora: The World of AVATAR and Star Wars Land projects debut. I would certainly hope so.