I'm another one who actually kinda liked Sub Terra and think that whole fiasco is a shame... It was a stupid idea (as was Derren Brown, more so) to pour that kind of money into a dark ride for an audience that just doesn't care. And it was finished off by budget cuts and I thiiiiink there was some post-Smiler issue with evacuation procedures, but I might be wrong. I vaguely remember hearing a rumour about it, can't remember if it was in person or I read it, but it makes sense. A lot of rides closed at Merlin parks shortly following Smiler to put in evacuation platforms.
But my real vote goes to poor DarKastle, that was forever in the shadow of Spiderman and later Transformers. For a regional park like Busch, it was bloody good and I unreasonably livid about it closing. Especially because they're not replacing it with a ride but yet MORE bs entertainment venue space. Like Drachen Fire's field. Piss off BGW. Piss right off.