I know for a fact that BoulderDash was a challenge logistically because of its mountainside location. Just surveying for the ride had to be difficult enough with all the trees and boulders in the way. Then to pump concrete and get supplies, men, and equipment up to the actual worksite becomes an almost daunting task.
Anytime you have to deal with irregular terrain with/and/or existing obstacles to deal with, the project becomes more challenging. Especially when there is no easy access to the site.
I agree with Boulder Dash being difficult to build...
Another one which is currently being built which I would imagine to be difficult would be Storm Runner at HP. I mean, It interacts with so many of the other rides and stuff in the park. While we're in Hershey, i think Great Bear would've been hard. Its side by side to Super Dooper Looper and on top of the log flume and flies over the midway and over the water. There are some supports on Great Bear I have never ever seen on any other inverted coaster.
And Dorneys new one for 2005 sounds like its gonna be difficult, going below ground level 5 times and right next to the lake. I think most coasters over water would be hard, like Anaconda at PKD.....I wondr how deep the water really is though.......
I guess terrain coasters are the hardest to construct. I always wondered about Dragon Mountain at Marine Land. That is a some construction marvel.
From a design standpoint my walnut brain thinks deulling dragons was the hardest to design and execute. I don't even want to think of the dynamic equations they used to make sure that two coasters with two separate layouts interact.
Obviously they had a lot of problems with Son Of Beast since it partially collapsed during construction.
I would also think that Hoosier Hurricane and Cornball express were special challenges because of how much track is over water. The tight turns both verticaly and horizontally on LoCoSuMo were probably a challenge too.
I got to listen to an interesting engineering seminar on the construction of the two 'new' coasters at SFNO this year. The entire park, including the two new coasters and the new park section, is built on a swamp. Everything from footers to the midways at the park have several thousand pilings. It may not look like anything special, but a lot of complex construction and design went into this park.