I'll do one for each park because I can't pick just one. I thought I'd already posted in this thread, but I can't find it. So forgive me if this is a double post.
Magic Kingdom
I love Splash, Space, the Laugh Floor, Haunted Mansion, and just about every other attraction in this park. However, the quintessential WDW attraction for me is Philharmagic. It embodies all of the Disney Magic.
Epcot
I love Soarin' and Test Track. They are just great fun. However, I'm going to go with something much more simple. Norway - the whole pavillion. I have so many wonderful memories of it with my husband. Ironically, this used to be my least favorite.
MGM
This is a toughie. Aerosmith was very first Disney ride so it will always hold a special place in my heart. However, I can't call it my favorite. Toy Story is just a blast, but, again, it doesn't earn that favorite status. Before I made my first trip to the World in 2005, I watched Disney specials for hours getting ready for my big day. The ride I most wanted to experience was the Tower.
AK
I love this park, especially at night. No particular attraction makes it the best. For me, it's one simple element that stands out as my favorite - theming.