Oooooh my old topic's got some activity!
I realise I never actually gave my own conclusive answer. I guess if I had to choose one I'd go for Viking's River Splash. Fantastic themed ride, something that is (in my opinion) completely ahead of anything in the UK in terms of rapids. One complaint would be the trough, but then its obnoxious trough is sort of part of the experience now.
The Dragon is a strong family dark-ride/coaster, not really like anything I've been on, so it would probably come in second for me. The dark-ride section is wonderful, so much attention to detail. The outdoor section is fairly thrilling for a coaster of such a low height restriction. With such a beautiful plaza, even queuing for this ride (the queues were its main drawback) was never much of a bore.
S.Q.U.I.D. Surfer comes in at a very close third. Such a bizzaire, unique ride unlinke anything I've ever experienced. A very nostalgic attration for me as I remember it so clearly from my annual trips in the 00s (when it was Wave Surfer).
I haven't done Pirate Falls since its retheme in 2014, but I did rather like it as a kid. It was my first ever log flume and I loved it despite its slowness. Main problem with this one was my memories of queuing up to an hour for this average log flume in March. The waits were insane (but at least you could play with Lego).
I've never understood Atlantis. I think I somewhat missed the target age group (being 10 when it opened, I was a bit older than its target audience, but not old enough to appreciate the technology etc). The hype surrounding the ride led me to queue 80 minutes for it during opening year and it was just a bit uninteresting. I was far beyond the age of believing that we were actually heading underwater and the fish didn't fascinate me that much. I've never seen why I'd go and queue for this when I could take my annual pass down the road to Chessington whenever I want and walk aound the SEALIFE centre at my own pace without queues.