Thunder Dolphin at LaQua is a beautiful coaster in a dramatic setting. There's something just right about its proportions and it doesn't suffer from ugly supports which is what holds back a lot of Intamin coasters in the beauty stakes. I also love the way it interacts with the ferris wheel and the shopping centre.
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I can't believe no-one's mentioned Air yet. Apart from the greenery that has started to grow on the track, its a beautiful coaster with smooth flowing lines, a beautiful setting and a great colour scheme. In fact, add all B&Ms to the list, I can't think of one I don't like.
GCI is pretty much the same, I can't think of an ugly coaster made by them either.
There's probably more but I can't think of them right now