Scariest would be the Supreme Scream at Knott's
http://www.knotts.com/park/tour/brdwalk/sscream.shtml
You go up
slowly over 250 feet and then they leave you up there for a while before you drop down at 50 mph. I'm afraid of heights and this didn't help. I did it once and I will never ever get back on that thing.
As for the wildest ride, that would be X at 6-flags Magic Mountain. That roller coaster alone is great, but add seats that independely rotate 360 degrees and you are in for the craziest ride of your life.
If the seats thing doesn't make sense, check out this diagram:
http://www.ultimaterollercoaster.com/coasters/reviews/x/x_sfmm_train.shtml
Imagine, for example, going into a loop looking forward, and coming out backwards, head down. Amazing.