I love hardstyle and I love psychedelics, but combining the two? whew I'm really not sure if I'd want to do that. Strenuous physical activity, sensory overload, a sea of people, I would worry I'd freak out or overexert myself, or get dehydrated while out of my gourd.
If you'll be "dancing" (hakkuh/jumpen) most of the time I'd say something that doesnt interfere too much with your hydration and exertion. A low dose of shrooms comes to mind. If you'll be mostly roaming the crowd you might try something more stimulating, such as one of the amphetamine phenethylamines (DOC, TMA's, 3CE).
So if you are hell bent on combining a hardstyle festival with psychedelics, use a physically benign one like shrooms or LSD if you want to go physically wild and have a more stimulating one if you want to go mentally wild.
Oh, and bring earplugs and sunglasses so that if it gets too sensorily or socially intense for you on psychedelics, you can cocoon up by turning the lights, sound and social scrutiny down. If you have them -- take benzos with you.