Whichever one appeals to you the most. Typically, MK tends to be crowded on Saturdays. It is usually fairly quiet on Sundays.
A common pattern, IMO, that has been common for many years is that people go to MK first, Epcot 2nd, then HS and/or AK. Their first full day tends to be Saturday. On their 5th day, they return to MK , and maybe go to Universal late in the week.
Universal is decidedly more crowded Thurs, Fri, Sat.
If you want to avoid crowds, generally try a different rotation, but that would maybe mean not going to Mk on your first day.
Special events, park hours, and weather also have an impact. People will go to MK on a day it has extended hours, but if you can't park hop then MK on a short hour day is a big sacrifice. So you might have to decide between getting longer park hours and/or lower crowds. If you can stay out late, the late night hours can be great. If you know you don't want to be out at night, a day when MK closes at 6pm can be great. It depends on your group.
HS always seems to be a mess these days. If any ride goes down, the park gets gummed up in a major way.
Braving rain or cold can also = lower crowds.
All of this brings me back to my original thought though, pick the park you want to visit. Crowd levels will only vary so much.