I've gone to WDW solo at least 20 times. However, the shortest stay was probably about 6 days!
If I just had one day, I would choose one park and do it all. Although I like MK, if I just had one day, that would not be my park of choice. I would choose from the other three at the last moment depending on what I felt like that day. I do that often when I go, just follow where the spirit leads for each day.
Epcot and MGM are both great parks for spending all day and just seeing it all. Look for the street performers at either park and watch them. Most people really miss those types of things by having to hurry from one attraction to the other. If rides are what you enjoy, do them as many times as you like.
I love AK as well. I can spend a full day there and more. The shows are great and the jungle trek and forest walk are wonderful. The longer you spend on them, the better they are. Most people just zip through and miss so much. The afternoon parade is great as well. If you start with AK, you will still have the chance to do either Epcot or MGM later as AK closes early.
Stop and talk to the CM's. They are great to talk to and always have great stories to tell. Even if you go to AK, end the park portion of your day at either Epcot or MGM with Illuminations or with Fantasmic! The main thing is to look at the details. Stop and look at the themeing of the park. Check out the detail in the flowers. Notice how each country in the WS has different lamp posts that fit their country and watch how it transitions from one to the other. Pull the rope outside of Indiana Jones. Look for the hidden trail at AK. Stop and watch all those people that are pulling their hair out trying to figure out how to see this and then get over to the other side of the park in time for that and then back for the other (and smile because you are just enjoying the day).
I agree with DeeP, after the parks close, head to DD and/or PI. I go over to one, or both almost every night of my stay. Extends the day and is a lot of fun. Sometimes I just wander the stores or PI. Other times I go for something specific. I love the Adventurers Club.