Overall, Magic Kingdom is my favorite park. But that's mainly because there are so many family friendly attractions.
If I was going to spend a day at an Orlando park by myself, I would probably pick one of these:
Epcot
Disney Hollywood Studios
Sea World
Epcot is a cool place to eat, drink, and just walk around and hang out. Plus, it has Soarin and Mission:Space, which are both a lot of fun.
Disney Hollywood Studios has two awesome rides (Rock n Roller Coaster, Tower of Terror) and some of the best shows (Indiana Jones, Lights, Motors, Action, and Fantasmic)
Sea World is a more relaxed park, with a fantastic coaster (Kraken) and all the whale and dolphin shows.
Normally, crowd levels at each park are higher on days that have EMH (Extra magic hours perk for people staying at a Disney resort). The EMH schedule plays a much bigger role than what day of the week it is. But the Food and Wine festival draws a lot of locals, so weekends at Epcot during that time will definitely be crowded.
It really just depends on what you are most interested in. Check out the Disneyworld website to look at which shows are offered which night at each park.