For me it has to be all the old Schwarzkopfs from the UK. This is not because I have rode them and think that I would love to ride them again, but because I was too young at the time to ride them I haven't (god how I wish I was born 10 years earlier)!
Would have honestly loved Towers and Flamingo in the 90s... they looked amazing!
The one coaster I miss the most (that I have been on) is probably Thunder Mountain at Flamingo Land, just for the reason that it was my first "proper" coaster really. I was heartbroken as a kid when I turned up at the park in 2005 to find it had gone!
I also had a soft spot for the Gold Mine at Blackpool Pleasure Beach, however its replacement isn't too bad