Ok, MK is nearly impossible to pick: Small World (because it's my girls favorite and I'll never forget their first ride on it), Pirates, Splash. And it's not a ride, but the Castle - brings me to tears more than once a trip.
Epcot: FW - Soarin, WS - the Mexico boat ride. I know, big dork, but that was officially the first ride my kids ever did at WDW and for some reason as a kid I always got such a kick out of it!
MGM: Again, not a ride, but the Osborne Family Spectacle of Lights - just way too cool.
AK: KS - though I love EE, KS is the only time I get to do a ride of that type.
This was fun!