Wipeout (blue metal pipework) is the worst, a dual-direction rollercoaster, takes you upside-down a few times and is very sickly on the (in-reverse!) return leg.
Enigma (red metal pipework) is quite tough with very tight turns, and stops very hard at the end.
Snake in the Grass is just a bit of fun but gets a few screams. When we went on it in the rain it struggled to climb up the starting ramp, and we were sat there for a minute wondering if we would need to be rescued!
Here's a video of Wipeout: