Only ever been to four but I don't suppose they will be a patch on anything in America.
Aqualand in Tenerife, very hilly.
Port Aventura WaterWorld in Salou. Okay but water was freezing
Aqualandia, Benidorm. Great, although all the slides are quite a walk away from each other and you have to pay for sunbeds and rubber rings.
Waterworld, Ayai Napa. FANTASTIC, all rides within easy walking distance, don't have to pay extra for beds, rubber rings, food pretty reasonable and plenty of lifeguards.