I'd rank them as follows, from least-most scary
BTM
Indy
Crush
R&RC
Space Mountain
I put SM last because its so rattly and boneshaky now that I've gone right off it, I find it very scary simply because it makes me feel rough. Very rough. Headachey and bleugh.
Since R&RC opened that is in my opinion the 'best' coaster. Very smooth, excellent theming, great track, love the music. Yes, its perhaps frightening to watch, but its less scary once you're actually on it.
Crush is fantastic, the spinning is a bit disorienting but very very good fun.
Indy - a 'pimped up BTM' - the best description I've ever seen. Not scary, not really, one loop and one major drop, just close your eyes and then its over!
Our rule - ride it once, even if you hate it ride it again, then after that if you still hate it then you dont' have to ride it.
I did this with Indy when it went backwards - I had to lie on the floor and hold on afterwards, to the extent that a CM came to see if I was alright. It messed with my head going backwards, so I never rode that one again all the while it went backwards. Now it goes forwards I love it!
ETA: BTM remains my absolute favourite. Well perhaps that and ToT.