Definitly try to give EPCOT a full day... I have never heard that acryonm about Every One Comes out Tired, but its SO TRUE! Be sure if you are going during peak season and want to eat at a World Showcase restaurant to make a reservation. This is one of the few places I highly recommend it at!
You will want to make sure you have a fast pass for Soarin', and also possibly for Test Track. In all the times I've been there I have never seen Mission Space with a long line.
Magic Kingdom probably will take atleast a full day to cover the major rides. The must hits in my book are the mountains (Splash, Big Thunder and Space), Haunted Mansion, Pirates of the Carribbean (it will be down for re-hab from March to July though), Jungle Cruise, Buzz Lightyear and Peter Pan's Flight. I also really like the Tommorowland Transit Authority for resting the feet!
MGM can usually be done in half a day if you hurry, but its a good idea for a full day too. Its not as big as the other parks. The big rides are Tower of Terror and Rockin' Roller Coaster. Make sure you get a fast pass for TOT! Great Move Ride, Stunt Show, Muppet Vision, and Star Tours are also very good attractions/rides. If you can I would recommend getting an ADR for a Fantasmic show package (available at Brown Derby, Mama Melrose and 50's Prime Time), this gets you priority seating for Fantasmic and its really worth it.
AK really should have a full day to fully appreaciate the whole park. If you just want to ride the rides though you could probably do it in half a day. DEFINITLY ride Kilimajaro Safaris (use Fast Pass!), Expedition Everest and the Festival of the Lion King, these are the best attractions at AK.
Also if you are looking for a really nice romantic dinner, we celebrated our anniversary this year at California Grill at Contemporary. Its VERY pricey, but if you can get a window seat at the right time it has a spectacular view of the MK fireworks.
Jennifer