SOOOO many choices so here goes.....
Corkscrew.... even though it was old and rough it made alton towers and always featured on the old Logos.... (i may upset a few people here but id gladly pull down Th13teen and get old Corky back.
Toyland Touts.... the music is just so damn catchy to the point where im humming it on some days....(Goes to youtube now im thinking of the music
The Alton Beast/Blackhole..... really enjoyed both of these coasters and its weird going to X-Sector now and not seeing the tent that has been there for all the time i can remeber.
The Alton Swans.... i know it wasn't fast or intense.... that ride was an icon of Alton Towers in the l90's and early 2000's me and my previous partner used to just go on it .... really gutted when it left and was replaced my the money grabbing PANT$ KART$ (sorry Splash Karts).... it was then the some of the magic of alton died in my opinion but times change i guess.
If i had to choose one it has to be........Toyland Tours im still humming the tune even now typing this