Our first trip was when my kids were 5, and we did three days at MK and one at AK. The first day was a half day at MK, then a full day at MK, a shorter day at AK, and our last day at MK again. It is totally a matter of preference, but there was plenty to do at MK to fill the two and a half days we were there, and since none of us had ever been to HS and Epcot before, we didn't miss them. We went again when they were 6 and did a full day at MK, half a day at MK before hopping to AK for the Ultimate Night of Adventure VIP tour, and then the Ultimate Day of Thrills VIP Tour, which hits the headliners at MK and HS as well as Soarin' at Epcot, on our last day before hopping back to MK. It was a good mix for us since the tours let us do everything we wanted to do at AK and HS in a short amount of time, but this approach requires hopping. We went in September of this year and did a half day at MK, a full day at AK, a full day at HS, and a full day at MK. Had hopping been available, we probably would have ended up at MK every night. We have yet to go to Epcot other than during the Ultimate tour, which hardly counts since they take you in the back way to ride Soarin' and then leave, but we are looking forward to adding it on the next trip.
For us, MK is where we like to spend most of our time, and we like to start and end our trip at MK. By skipping HS and Epcot on our first trip, we had a new experience (HS) on our second trip and will have another new experience (Epcot) on our next trip. We are big ride people, so we usually skip the shows and even the animal paths at AK. Some of our favorite rides are at AK and HS, but you cannot beat MK for the sheer number of rides and for classic Disney magic.