Let me be the first to say the twilight zone tower of terror. It's just that simple, and i know that this forum will be chock full of the tower of terror within 2 days. secondly it's Rock and roller coaster, and then thirdly there is Splash Mountain.
Rock N Rollercoaster gets boring after the launch...(In my opinion)...the Launch in RNR is the greatest "thrill" in all of WDW though. But for overall ride quality...It's a tie between Splash and ToT...just because the theming is so unbelievable in Splash but the thrill is so much greater in ToT...
My #1 thrill ride in WDW : Tower of Terror
(but we'll see what Mission:Space brings)