I believe Phantasialand aims for every 3 years but unsure on that. Merlin can be quite good on the large investments every so often but never anything of significance between the investments, tend to agree with @MouseAT here.
Probably the Dorney Park Rapids. Went on only expecting a light dusting as you would get on UK rapids, I was wrong! Course itself fairly tame but all hell breaks loose with fountains and waterfalls!
Honorable mentions to Valhalla and Walibi Belgium rapids