Magic Kingdom
Most Underrated: Carousel of Progress - such a lovely piece of entertainment. Sure, it's old, the last scene needs updating, and it doesn't appeal to most people under 30, but it has history and heart. The characters are lovable, charming and easy to listen to, the sets are a feast for the eyes, and the ride system is ingenious. I can tell why Walt loved this one.
Most Overrated: Buzz Lightyear's Space Ranger Spin - man, I loved this ride until I visited Uni for the first time and took a spin on MIB. This ride pales in comparison and shows its true colors - a cheap retrofit of an existing space. Way too many cutouts for the ride to have any true depth (both literally and figuratively). The plot is cute and done well enough, but Disney could do so much more with this type of attraction (and no, I don't mean Midway Mania). Oh, and the bolted down guns blow!
Epcot
Most Underrated: Living with the Land, I guess. It's a decent attraction that acts as good filler and includes some fun education. I really picked this one out of lack of any other choices... many of Epcot's attractions are sadly overrated or otherwise mediocre.
Most Overrated: Soarin`. This is seriously considered an E-Ticket, the "top" ride of the park? Cmon. It's a very, very bare bones simulator - and it doesn't do a very good job hiding that it's just that, a simulator. There's very little excitement once your in the air, you're just staring at a blurry/dusty/old projected image of California. The concept is great, but the execution is minimal and leaves much to be desired. Oh, and the California film instead of a new one (say, around the USA, or around the world?) Pretty lame...
Disney's Hollywood Studios
Most Underrated: The Great Movie Ride, easily. This ride is on such a grand scale that I fall in love with it every time. The films are well chosen, the driver/hijacking/audience interaction is fun, the animatronics are a marvel (especially Wicked Witch)... the pacing may be a little slow, but I love that the ride is so lengthy - allows you plenty of time to savor the spectacle around you. It's a huge shame the ride is so hidden and even bemoaned by some fans as "boring" or "dated..."
Most Overrated: Rock'n'Roller Coaster. This ride is all too short with some pretty flimsy theming. I don't get why the pixie dusters harp on Uni for its coaster rides when they have this "gem" sitting in their backyard... sure, it might be indoors, but it's still just a roller coaster in dim light with some cheap flats. Is it fun? Sure. Is it worthy of its "best ride in the park" status? No way.
Disney's Animal Kingdom
Most Underrated: Dinosaur. There frankly aren't a lot of attractions to choose from. I really hate how Dinosaur's been toned down, with plenty of special effects removed, and even plenty of sets never installed due to DAK's accelerated opening... but that doesn't stop Dinosaur from being one hell of a ride. Plenty of amazing animatronics to see here with plenty of thrill... that, and for a Disney ride it's surprisingly scary. The plot's intensity is in your face and never lets up... the story overall is solid and well executed. Right now it's a very good attraction... it could be one of the best rides ever if it had reached its full potential.
Most Overrated: Expedition Everest. Frankly, I think you have a problem when you can count the ride's special effects on two hands... and can count the working ones on one. The focus seemed to be on the research for the queue and the facade of the mountain... which is great and all, but the ride is where it counts, and it underwhelms. I won't even get into the Yeti - even when he was working (and I saw it several times), while impressive, it didn't save this relative disappointment of a ride.
Universal Studios Florida
Most Underrated: E.T. Adventure - the thought of this ever leaving makes me so upset. ET is cornerstone of classic cinema and is represented so well through this attraction... it has an amazing original score, some jaw-dropping sets, beautiful flora/fauna/creatures, and is honestly just such an experience. It needs a good refurbishment to modernize the police officers/fix up some of the aliens, but the concept here is timeless. The moment you arrive on The Green Planet, with the enormous rock work, deafening steam, glowing magma, and foreboding score... it's one of my favorite moments in theme park history. Subtlety is lost on current attractions, this one has it. They don't make rides like this anymore.
Most Overrated: Shrek 4-D. I visited this recently during an early entry period and I shake my head at the absolute laziness of this attraction. The continuity of the film's plot and character motivations is broken clean in two, and the transition from the preshow to the main show doesn't even make sense. The film itself is amusing and has its moments, and I don't mind the loud moving chairs like some people do - my issue is with the total non-involvement of the audience. The theater is as bare as can be and contains plenty of contradictory theming (why is there a big Shrek banner in Farquaad's torture chamber?) I just go through this attraction and think to myself, we lost Hitchcock to this? I can't believe this is used as an advertising point for the park.
Islands of Adventure
Most Underrated: The Cat in the Hat - while it isn't as masterful as ET, Cat in the Hat is an extremely well executed dark ride that I feel doesn't get enough positive attention. The pacing and styling of the story is pitch perfect, it has plenty of delightful visual gags, sets, and animatronics, and it's a ride for the whole family - a class of rides in short supply at Islands of Adventure. I wish they'd fix some of the AAs (the mouths always seem to be a trouble spot), but otherwise this attraction is a real winner.
Most Overrated: Dudley Do-Right's Ripsaw Falls - this ride is a mess. The story has promise if you pay razor sharp attention in the queue, but none of it is followed up on in the ride itself... none of the scenes really connect together. I hate the argument that it's "supposed" to look cheap and cheesy, because it's that mindset that gave us such wonders as Chester & Hester's Dino-Rama. There was an opportunity here for a really cool flume ride that offered a quirky style with a little-seen cartoon character, but instead they cheapened out and gave us a minimalist theme park effort. The amount of decay, lack of maintenance, peeling paint, backstage areas, poor lighting and staging you see throughout this ride is downright depressing. At least the facade is beautiful - that is an achievement. Unfortunately, it seems all the focus and money went to that and not the ride itself.
Busch Gardens Tampa
Most Underrated: Gwazi - Okay, hear me out. Even though some of you will be terribly confused given my "Most Overrated" choice for this park, I have to pick Gwazi as my underrated - simply because while it's rough, I find it to be in the "good rough" class. I ride Gwazi and it kicks my ass - but it feels more like a positive achievement than a survival one. It might very well be the restraint system - there's nothing to bang my head on as I get whipped around. Gwazi's layout is killer and both tracks are great. I actually haven't tried the newer trains... maybe that'll make the ride even better? (For what it's worth, I rode the infamous Mean Streak @ Cedar Point plenty of times and never had a problem. I guess I just handle wooden coasters well?)
Most Overrated: Kumba - I really wanted to enjoy this coaster as the layout is pretty spectacular, but on all my rides I've been beaten to hell and back. I've tried sitting in every spot on the train, but it's always so rough! I seriously can't handle more than one ride per trip or I'll get a serious headache. Again, the layout is great (though not nearly as exciting as Montu), but I just can't handle the ride's fierceness for some reason...