In recent memory, Dragon Kahn. I thought it was good, but I thought Furious Baco was much better. However, everytime I rode Kahn it got better, i.e. from the back and at night time, but I still think it's overated.
Nemesis. It's good, but come on. Some people seem to think Nemesis is God.
Tatsu. When I rode this, I thought "is it actually possible to make an exciting B&M Flyer?" The pretzel loop is great, but the rest was just dire. But the hype it has recieved from enthusiasts was sensational.
Millenium Force - ish. I thought it was so much fun, but again, the hype it recieves I expected to be blown away, I wasn't. I try to take every coaster for what it is, ignoring expectations, but the hype this thing recieves I couldn't help but feel a little gutted.
N sorry fanboys, but Jubilee Odyssey. I seriously have no idea why people love that thing.
Probably a few others as well, but I feel these are the ones that stand out most.