I would have to vote for EPCOT also if you have to limit it to one. We had Park Hopper passes when we went in March and it worked out great. EPCOT is definitely a lot to cover in one day.
With a 4 and 5 year old, you have to go to Living Seas and check out Turtle Talk with Crush! It's great! My 2 1/2 year old was laughing hysterically and my 9 and 12 year olds loved it too. I think everyone in the show was having a blast!

The way Crush interacts with the audience is great!
Also, the pressed penny machines are wonderful! Cheap souvenirs and the kids have fun finding the penny machines in each country
Check the times on the schedule for the Character greeting bus. A big bus stops at different spots throughout the day and has a LOT of characters on it. When we were there, Mickey, Minnie, Donald, Pluto, Goofy, Rafiki, Max and a few others were there. We weren't able to see all of them, but made it to quite a few of them

Our DD2 was in heaven!
Another thing my DS9 did was to get "autographs" in his autograph book from a cast member in each country. He had them write in their language and then we wrote in the translation right away. He had a good time doing that and the cast members, for the most part, seemed excited to be asked
Whatever you do, it will be fun

It is Disney after all!! Have fun!