We took our daughter at 14 mos. (A few notes to consider, she's not afraid of the dark at all, doesn't really mind scary things, and loud noises barely phase her, she was just pretty much beginning to walk and absolutely ADORES music and disney characters)
The biggest hits for her were:
Character Greetings in every park- she literally cried when we would take her AWAY (and dive out of our arms waiting for them trying to get to Mickey, Minnie etc..) Her second word though was "ickey" for Mickey so that should explain a lot.
MK
Top rides/attractions
Buzz Lightyear - we literally rode 3-4 times. She loved holding the laser gun and pressing buttons and spinning the car.
Dumbo - she actually loved watching the train go by next door and the music while waiting. The play area was actually a little difficult for her as she was still pretty "toddler-y"
The Carousel
The Little Mermaid
The Enchanted Tiki Room
Electric Light Parade (LOVED, loved, loved!)
Cinnamon rolls at Gastons and Dole Whip in Adventureland (yes, this was totally an "attraction" to my food loving child)
Good Rides/attractions - Everything went fine, she liked them (just wasn't necessarily bouncing off the walls excited like she was on the "top rides")
Small World
Peter Pan
Winnie the Pooh
Aladdins Magic Carpets
Jungle Cruise
Pirates of the Caribbean (just hang on to the little one tight during the drop)
Tea Cups
WDW Railroad
People Mover
Philharmagic surprisingly kept her interest well.
OK attractions - A little squirmy
Country Bear Jamboree- We thought she'd like this more but forgot how not so catchy the music was.
Princess and Tinkerbell greeting- the actual people weren't that interesting to her. We have a picture of her looking at Cinderella with one raised eyebrow and it is the funniest thing ever.
Haunted Mansion - we went on too close to nap time
Animal Kingdom
She LOVED the Festival of the Lion King. A little squirmy waiting. We requested to sit near an exit just in case she started crying or something and we had to leave and they were totally accommodating (great way to get a good view btw) but once the show started she was enraptured!
Love the Safari Expedition too.
The Mickey and Minnie greeting had almost no wait.
Seeing the animals was fine, but she mostly fell asleep and napped in her stroller.
Rafiki Planet Watch was a hit (and the train ride out there too). She loved petting the goats and the snake too! She also loved Rafiki and watching the kids interact with the Chip and Dale sing along.
Was totally unimpressed eating at Rainforest (which surprised me)
We totally forgot all about the triceratop spin, and had to miss our Nemo FP as she was asleep in the stroller.
Her favorite part though was sitting on the Elephant statues right when you got off the train at Rafiki's...go figure?!?
EPCOT
Rode Imagination, Mexico, Spaceship Earth, Living with the Land and even got through the American Adventure and France movie with no problem. She loved looking in the tanks at the Living Seas and the Nemo ride went smooth. She threw a FIT though waiting for Turtle Talk and we had to make a fast exit before it started. We got stuck in the back of the room and she couldn't see and all the kids had already filled the seating area up front and so I think she was mad that all she could see was the back of grownup heads and nothing was going on yet. Never could stay up late enough with her to catch Illuminations. Maelstrom was closed already when we went.
She was totally cool with riding around in the stroller checking out all the shops too.
We did the character meal at the Garden Grill here too and they were really accommodating with the little one. They even brought her the kids meal dish and cupcakes even though she was under 2 and we weren't even paying for a meal for her. It was reasonably calm too the way the tables are laid out in a circle there.
Studios
Playhouse Disney was a hit, but I had to work really hard to keep her from running all over the room before and during the show. The open carpet just said "run" to her. ugh.
The muppets was fun, but of course she wouldn't wear the glasses.
TSMM was fun but obviously we really couldn't play the game.
The only thing we didn't do a lot of were longer shows that didn't have easy exits e.g. The Great Movie Ride, Universe of Energy, as kiddo can get unpredictably squirmy and we didn't want to be stuck if she wasn't having any of it (e.g. throwing a fit. lol)