First, before jumping up to one of the big 3, I would consider riding Revenge of the Mummy or Flight of the Hippogriff first. Revenge of the Mummy is 1/2 coaster, 1/2 dark ride with a launch. Flight of the Hippogriff is a big kiddy coaster, think oversize barnstormer, but you go by the front of Hagrid's Hut (you sort of go by the back in line for Dragon Challenge) and you get a good areal view of Hogsmead from the top of the lift hill.
Once they get some "coaster legs" under them, it's a tough call on which one is the next scariest. Each one touches a different fear sensor.
HRRR is the tallest and has the vertical lift but the rides doesn't go upside down, you travel on the "outside" of the loop. If you can get past the beginning of the ride and it's disconcerting vertical lift, the rest of the ride reminds my slightly of bigger Everest after the projected Yeti. The Queue on HRRR is also the least interesting.
Dragon Challenge has 5 inversion on each side (red and blue are different coasters) which is fewer than the Hulk. But, being an inverted coaster, most of the seats can't see the track in front of them to "prepare" for what's coming next, and most of the maneuvers feel more "whippy" than rides with the track on the bottom. Dragon Challenge probably has the best of the 3 queues (as mentioned above) although I preferred the Queue when it was Dueling Dragons.
Hulk, is the one that's normally advertised as being the "big daddy" of coasters in the park. It has 7 inversions including a launch up the first hill into the initial zero G roll. It is also listed as the fastest of the rides. But, in many ways, it's not as intense as Dragon Challenge. Instead of having small "whippy" inversions, many of them are large and flowing. Hulks queue is nicely themed but nothing compared to the fantastic queues in WWOHP.