Valhalla at Blackpool Pleasure Beach.
I first rode in 2004 and it was like nothing else on the planet back then, the huge number of special effects, fire, ice, live electricity and water combined with the stunning soundtrack/sound effects made it feel like a fully themed experience with no areas cut back on.
The exterior was stunning with the imposing facade and skull (the original rockwork was hand-sculpted and was based on some cliffs in Yorkshire), original authentically crafted station and shop with the perpex wall allowing you to see the water falling down behind it, the window to watch boats exit through the shop, all the station crows were animated and the soundtrack played through a much better sound system.
Inside, moments like the first lifthill used to be almost show-like with the spinning tunnels, viking chanting and Odin lighting up as you approached, speaking in a deafeningly loud voice and then transforming into a skull as you started to crest the lift. He's been on a clumsily-edited loop for years now sadly, you can barely even hear him anymore.
The room before the final drop was incredible with the original thunder sound effect accompanied by plunging riders into darkness before the tesla coil kicked in and filled the room with volts of electricity.
Small moments such as a lighting effect to distract riders from the rolling log until it was near enough to surprise made all the difference, a level of showmanship that just isn't seen on the ride today.
The ride in its 2004 form made perfect sense as a themed experience, whilst it didn't explicitly explain the storyline it told the story through physical scenes whilst leaving some of it to the imagination. I'd love to be able to see the ride in its 2000 form as I believe there were even more incredible effects (such as a water screen) which only lasted 1/2 seasons before being removed.
Sadly the ride has suffered from modifications which take it further away from the originally intended experience, a lack of scenic care in many areas and a number of effects are either gone entirely, unreliable or working in a much less impressive way than previously.