I agree, it does depend on the ages, our children are 8 and 11, and the last couple years our favorites are:
Epcot-- Soarin' and Test Track and doing the passports at the World Showcase, Spaceship Earth (now that it has been redone), Ellen's Energy Adventure
Magic Kingdom--Big Thunder Mountain, Mickey's Philharmonic, Monsters Inc, Stitch, Buzz Lightyear, Pirates of the Caribbean, Haunted Mansion
Animal Kingdom- It's Tough to be a Bug, my children love the bone yard, Everest, Kilimingaro Safari, Kali Rapids (you do get pretty wet though!), Dinosaur
Hollywood Studios-The Great Movie Ride, Toy Story Mania, Tower of Terror, Rock 'n' Roll Rollercoaster, Indiana Jones Stunt Show
Have a great first trip! You will be hooked like the rest of us!!