Tough one this. Out of the three I've done the Liseberg one is my favourite due to how high up the cliff is. Being in such a tiny ride unit at that height spinning like a maniac made me feel so vulnerable and I came off shaking like a leaf. A ride hasn't made me feel like that since I first rode Stealth when I was about twelve so it was a pretty special experience and so unexpected from a gentle looking flat.
As a design though I prefer the Sky Fly because when you're spinning on the downward bit it really does feel like you're going to smack into the ground which I love. And the Blackpool one is great, such a fab idea to put it on the coast for maximum wind.
So yeah, the Sky Roller at Liseberg is my favourite one but if you were to plonk both models in a car park I'd rather ride a Sky Fly.