We hit tomorrowland first then head to frontierland if we are by ourselves. If our daughter is with us then, we head to fantasyland first and then toontown.
On a non-EE day, we pick up a FP for JC on the way to SM. So, technically we go to Adventureland first, but I answered Frontierland, because that's the first thing we ride.
Liberty Square Because ... I usually go to the Haunted Mansion first for several reasons: (1) because it gets so crowded later in the day. (2) it seems to break down and be out of service more often than any other ride in the park. (3) One can usually ride it two or three times with no waiting at all because most everyone else is at Space Mountain or Splash Mountain early in the morning. (4) It's long been my favorite attraction at the Magic Kingdom.
I might stop at the Jungle Cruise boat ride first, but for some reason it does not open until after the park has been open for an hour or so.
We hit Tommorowland 1st thing on 1st day in MK.
My DD (the daredevil) & I hit Space Mnt, and my son and my mom go to Buzz Lightyear, after Space Mnt my DD & meet at Buzz and ride along with them on their 2nd (or 3rd) time.
I voted for Liberty Square because we have always headed to Haunted Mansion first... This next trip might be different because the 5 year old will be our boss!
We make a quick bee-line to Fantasyland and ride Dumbo. We show the kids the lay of the lands and they get revved up to ride everything else. We can do FL in 1 hour once Dumbo is out of the way!