It's all down to personal tastes and what you want out of a coaster. Nemesis, Oblivion and (bizarrely) the Smiler seem to be great Intimidators from talking to others or hearing what they say about them.
I don't think Oblivion needs any explanation as to why it's seen this way. Nemesis looks as intense off ride as it actually is on ride, if you step back and watch it, it does appear to traverse some very tight elements very quickly and very close to pathways (excellent design), even by invert standards. The Smiler, well I don't understand it as much, it seems to be the vertical lift hill for some bizarre reason. Other than that, I can only put it down to fear of inversions because I've never personally seen the Smiler as an intimidating coaster both on or off ride. But there are alot of people who are scared of it? Seems strange to me.