I don't really have a best or a worst for river rapids rides. Instead, I split them into two categories, above average and average.
Grizzly River Run (Disney's California Adventure)
Shipwreck Rapids (SeaWorld San Diego)
Bigfoot Rapids (Knott's Berry Farm)
Rip Roaring Rapids (California's Great America)
Roaring Rapids (Six Flags Magic Mountain)
White Water Safari (Six Flags Discovery Kingdom)
I could not pick a best from the two above average ones, but if I had to pick a worst I would probably say White Water Safari. The ride gets you plenty wet, but most of it comes from water cannons the guests activate and not from the ride itself (other than the waterfall).
Top 5 Steel: Bizarro (SFNE), Tatsu, Maverick, Top Thrill Dragster, Nitro
Top 5 Wood: El Toro, Voyage, Boulder Dash, Thunderhead, Ravine Flyer II
Top 3 Parks: Busch Gardens Williamsburg, Holiday World, Cedar Point
TPR Trips: IntimidaTour 2010, North East Trip 2011, LeviaTHON 2012